[builtins] Allow compiling the builtins without libc headers

[arichardson]

Summary

When compiled with -ffreestanding, we should not assume that headers
declaring functions such as abort() are available. While the compiler may
still emit calls to those functions [1], we should not require the headers
to build compiler-rt since that can result in a cyclic dependency graph:
The compiler-rt functions might be required to build libc.so, but the libc
headers such as stdlib.h might only be available once libc has been built.

[1] From https://gcc.gnu.org/onlinedocs/gcc/Standards.html:
GCC requires the freestanding environment provide memcpy, memmove,
memset and memcmp. Finally, if __builtin_trap is used, and the target
does not implement the trap pattern, then GCC emits a call to abort.

[phosek]

LGTM

compiler-rt/lib/builtins/atomic.c

 //  To avoid needing a per-object lock, this code allocates an array of
 //  locks and hashes the object pointers to find the one that it should use.
 //  For operations that must be atomic on two locations, the lower lock is
 //  always acquired first, to avoid deadlock.
 //
 //===----------------------------------------------------------------------===//
 
 #include <stdbool.h>
+#include <stddef.h>
 #include <stdint.h>
-#include <string.h>
 
 #include "assembly.h"
 
+// We use __builtin_mem* here to avoid dependencies on libc-provided headers.
+#define memcpy __builtin_memcpy
+#define memcmp __builtin_memcmp
+
 // Clang objects if you redefine a builtin.  This little hack allows us to
 // define a function with the same name as an intrinsic.
 #pragma redefine_extname __atomic_load_c SYMBOL_NAME(__atomic_load)
 #pragma redefine_extname __atomic_store_c SYMBOL_NAME(__atomic_store)
 #pragma redefine_extname __atomic_exchange_c SYMBOL_NAME(__atomic_exchange)
 #pragma redefine_extname __atomic_compare_exchange_c SYMBOL_NAME(              \
     __atomic_compare_exchange)
 #pragma redefine_extname __atomic_is_lock_free_c SYMBOL_NAME(                  \

compiler-rt/lib/builtins/clear_cache.c

 //===-- clear_cache.c - Implement __clear_cache ---------------------------===//
 //
 // 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 "int_lib.h"
+#if defined(__linux__)
 #include <assert.h>
+#endif
 #include <stddef.h>
 
 #if __APPLE__
 #include <libkern/OSCacheControl.h>
 #endif
 
 #if defined(_WIN32)
 // Forward declare Win32 APIs since the GCC mode driver does not handle the

compiler-rt/lib/builtins/int_util.c

 }
 
 #elif __APPLE__
 
 // from libSystem.dylib
 NORETURN extern void __assert_rtn(const char *func, const char *file, int line,
                                   const char *message);
 
-#ifndef _WIN32
 __attribute__((weak))
 __attribute__((visibility("hidden")))
-#endif
 void __compilerrt_abort_impl(const char *file, int line, const char *function) {
   __assert_rtn(function, file, line, "libcompiler_rt abort");
 }
 
-#elif __Fuchsia__
+#else
 
 #ifndef _WIN32
 __attribute__((weak))
 __attribute__((visibility("hidden")))
 #endif
 void __compilerrt_abort_impl(const char *file, int line, const char *function) {
+#if !__STDC_HOSTED__
+  // Avoid depending on libc when compiling with -ffreestanding.
   __builtin_trap();
-}
-
 #else
-
-// Get the system definition of abort()
-#include <stdlib.h>
-
-#ifndef _WIN32
-__attribute__((weak))
-__attribute__((visibility("hidden")))
+  __builtin_abort();
 #endif
-void __compilerrt_abort_impl(const char *file, int line, const char *function) {
-  abort();
 }
 
 #endif