[libc][NFC] Remove all Linux specific code to respective linux/ directories

[abrachet]

Summary

These were all the non OS agnostic implementations I could find in general directories.

Currently none of these functions are actually enabled, but for when they do it makes sense that they be in linux/ specific directories.

libc/src/assert/CMakeLists.txt

 add_entrypoint_object(
   __assert_fail
   SRCS
     __assert_fail.cpp
   HDRS
     __assert_fail.h
     assert.h
   DEPENDS
-    # These two dependencies are temporary and should be replaced by fprintf
-    # later.
     libc.src.__support.OSUtil.osutil
-    libc.include.sys_syscall
     libc.src.stdlib.abort
 )

libc/src/assert/__assert_fail.cpp

 //===-- Implementation of __assert_fail -----------------------------------===//
 //
 // 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/__support/OSUtil/io.h"
 #include "src/assert/__assert_fail.h"
 #include "src/stdlib/abort.h"
 
-// These includes are temporary.
-#include "include/sys/syscall.h"  // For syscall numbers.
-#include "src/__support/OSUtil/syscall.h" // For internal syscall function.
-
 namespace __llvm_libc {
 
-// This is just a temporary solution to make assert available to internal
-// llvm libc code. In the future writeToStderr will not exist and __assert_fail
-// will call fprintf(stderr, ...).
-static void write_to_stderr(const char *s) {
-  size_t length = 0;
-  for (const char *curr = s; *curr; ++curr, ++length);
-  __llvm_libc::syscall(SYS_write, 2, s, length);
-}
-
 LLVM_LIBC_FUNCTION(void, __assert_fail,
                    (const char *assertion, const char *file, unsigned line,
                     const char *function)) {
   write_to_stderr(file);
   write_to_stderr(": Assertion failed: '");
   write_to_stderr(assertion);
   write_to_stderr("' in function: '");
   write_to_stderr(function);
   write_to_stderr("'\n");
   __llvm_libc::abort();
 }
 
 } // namespace __llvm_libc

libc/src/stdlib/CMakeLists.txt

add_entrypoint_object(
   _Exit
   ALIAS
   DEPENDS
     .${LIBC_TARGET_OS}._Exit
 )
 
 # add_entrypoint_object(
 #   abort
-#   SRCS
-#     abort.cpp
-#   HDRS
-#     abort.h
+#   ALIAS
 #   DEPENDS
-#     libc.include.stdlib
-#     libc.src.signal.raise
-#     ._Exit
+#     .${LIBC_TARGET_OS}.abort
 # )

libc/src/stdlib/linux/CMakeLists.txt

 add_entrypoint_object(
   _Exit
   SRCS
     _Exit.cpp
   HDRS
     ../_Exit.h
   DEPENDS
     libc.include.sys_syscall
     libc.include.stdlib
     libc.src.__support.OSUtil.osutil
 )
+
+# add_entrypoint_object(
+#   abort
+#   SRCS
+#     abort.cpp
+#   HDRS
+#     ../abort.h
+#   DEPENDS
+#     libc.include.stdlib
+#     libc.src.signal.raise
+#     ._Exit
+# )