[libc] Fix swab placement

[Caslyn]

Summary

Per https://reviews.llvm.org/D147970#4256889, swab.cpp is moved out of
the /linux subdirectory and cmake specifications are amended to reflect
that swab is not OS-specific.

[mcgrathr]

lgtm

libc/config/linux/aarch64/entrypoints.txt

set(TARGET_LIBC_ENTRYPOINTS
     libc.src.unistd.linkat
     libc.src.unistd.lseek
     libc.src.unistd.pread
     libc.src.unistd.pwrite
     libc.src.unistd.read
     libc.src.unistd.readlink
     libc.src.unistd.readlinkat
     libc.src.unistd.rmdir
-    libc.src.unistd.swab
     libc.src.unistd.symlink
     libc.src.unistd.symlinkat
     libc.src.unistd.sysconf
     libc.src.unistd.truncate
     libc.src.unistd.unlink
     libc.src.unistd.unlinkat
     libc.src.unistd.write
 )
list(APPEND TARGET_LIBC_ENTRYPOINTS
     # unistd.h entrypoints
     libc.src.unistd.environ
     libc.src.unistd.execv
     libc.src.unistd.getopt
     libc.src.unistd.optarg
     libc.src.unistd.optind
     libc.src.unistd.optopt
     libc.src.unistd.opterr
+    libc.src.unistd.swab
 
     # sys/select.h entrypoints
     libc.src.sys.select.select
   )
 endif()
 
 set(TARGET_LLVMLIBC_ENTRYPOINTS
   ${TARGET_LIBC_ENTRYPOINTS}
   ${TARGET_LIBM_ENTRYPOINTS}
 )

libc/config/linux/riscv64/entrypoints.txt

set(TARGET_LIBC_ENTRYPOINTS
     libc.src.unistd.linkat
     libc.src.unistd.lseek
     libc.src.unistd.pread
     libc.src.unistd.pwrite
     libc.src.unistd.read
     libc.src.unistd.readlink
     libc.src.unistd.readlinkat
     libc.src.unistd.rmdir
-    libc.src.unistd.swab
     libc.src.unistd.symlink
     libc.src.unistd.symlinkat
     libc.src.unistd.sysconf
     libc.src.unistd.truncate
     libc.src.unistd.unlink
     libc.src.unistd.unlinkat
     libc.src.unistd.write
 )
list(APPEND TARGET_LIBC_ENTRYPOINTS
     libc.src.unistd.execv
     libc.src.unistd.fork
     libc.src.unistd.__llvm_libc_syscall
     libc.src.unistd.getopt
     libc.src.unistd.optarg
     libc.src.unistd.optind
     libc.src.unistd.optopt
     libc.src.unistd.opterr
+    libc.src.unistd.swab
 
     # sys/select.h entrypoints
     libc.src.sys.select.select
   )
 endif()
 
 set(TARGET_LLVMLIBC_ENTRYPOINTS
   ${TARGET_LIBC_ENTRYPOINTS}
   ${TARGET_LIBM_ENTRYPOINTS}
 )

libc/config/linux/x86_64/entrypoints.txt

set(TARGET_LIBC_ENTRYPOINTS
     libc.src.unistd.linkat
     libc.src.unistd.lseek
     libc.src.unistd.pread
     libc.src.unistd.pwrite
     libc.src.unistd.read
     libc.src.unistd.readlink
     libc.src.unistd.readlinkat
     libc.src.unistd.rmdir
-    libc.src.unistd.swab
     libc.src.unistd.symlink
     libc.src.unistd.symlinkat
     libc.src.unistd.sysconf
     libc.src.unistd.truncate
     libc.src.unistd.unlink
     libc.src.unistd.unlinkat
     libc.src.unistd.write
 
list(APPEND TARGET_LIBC_ENTRYPOINTS
     libc.src.unistd.execv
     libc.src.unistd.fork
     libc.src.unistd.__llvm_libc_syscall
     libc.src.unistd.getopt
     libc.src.unistd.optarg
     libc.src.unistd.optind
     libc.src.unistd.optopt
     libc.src.unistd.opterr
+    libc.src.unistd.swab
 
     # sys/select.h entrypoints
     libc.src.sys.select.select
   )
 endif()
 
 set(TARGET_LLVMLIBC_ENTRYPOINTS
   ${TARGET_LIBC_ENTRYPOINTS}
   ${TARGET_LIBM_ENTRYPOINTS}
 )

libc/src/unistd/CMakeLists.txt

 add_entrypoint_object(
   rmdir
   ALIAS
   DEPENDS
     .${LIBC_TARGET_OS}.rmdir
 )
 
 add_entrypoint_object(
-  swab
-  ALIAS
-  DEPENDS
-    .${LIBC_TARGET_OS}.swab
-)
-
-add_entrypoint_object(
   symlink
   ALIAS
   DEPENDS
     .${LIBC_TARGET_OS}.symlink
 )
 
 add_entrypoint_object(
   symlinkat
add_entrypoint_object(
   DEPENDS
     libc.include.unistd
     libc.src.__support.CPP.optional
     libc.src.__support.CPP.string_view
     libc.src.__support.File.file
     libc.src.stdio.fprintf
 )
 
+add_entrypoint_object(
+  swab
+  SRCS
+    swab.cpp
+  HDRS
+    swab.h
+  DEPENDS
+    libc.include.unistd
+    libc.src.__support.CPP.optional
+)
+
 # These aren't actual external per-se, but this is just an easy way to create
 # targets that don't do anything. They exist to be referenced in entrypoints.txt
 # so that the header will properly expose their definitions. Their declaration
 # is in getopt.cpp.
 add_entrypoint_external(
   optarg
 )
 

libc/src/unistd/linux/CMakeLists.txt

DEPENDS
     libc.include.fcntl
     libc.include.unistd
     libc.include.sys_syscall
     libc.src.__support.OSUtil.osutil
     libc.src.errno.errno
 )
 
 add_entrypoint_object(
-  swab
-  SRCS
-    swab.cpp
-  HDRS
-    ../swab.h
-)
-
-add_entrypoint_object(
   symlink
   SRCS
     symlink.cpp
   HDRS
     ../symlink.h
   DEPENDS
     libc.include.fcntl
     libc.include.unistd

libc/src/unistd/swab.cpp

 //===-- Implementation of swab --------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "src/unistd/swab.h"
 
 #include "src/__support/common.h"
 
 namespace __llvm_libc {
 
 LLVM_LIBC_FUNCTION(void, swab,
                    (const void *__restrict from, void *__restrict to,
                     ssize_t n)) {
   const unsigned char *f = static_cast<const unsigned char *>(from);
   unsigned char *t = static_cast<unsigned char *>(to);
   for (ssize_t i = 1; i < n; i += 2) {
     t[i - 1] = f[i];
     t[i] = f[i - 1];
   }
 }
 
 } // namespace __llvm_libc