[compiler-rt] Fix atomic_test.c on macOS

[arichardson]

Summary

The macOS name mangling adds another underscore. Therefore, on macOS
the atomic_* functions are actually _atomic_* in libcompiler_rt.dylib.
To handle this case, prepend the asm() argument with USER_LABEL_PREFIX
in the same way that atomic.c does.

compiler-rt/test/builtins/Unit/atomic_test.c

 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
 // We directly test the library atomic functions, not using the C builtins. This
 // should avoid confounding factors, ensuring that we actually test the
 // functions themselves, regardless of how the builtins are lowered. We need to
 // use asm labels because we can't redeclare the builtins.
+// Note: we need to prepend an underscore to this name for e.g. macOS.
+#define _STRINGIFY(x) #x
+#define STRINGIFY(x) _STRINGIFY(x)
+#define EXTERNAL_NAME(name) asm(STRINGIFY(__USER_LABEL_PREFIX__) #name)
 
-void __atomic_load_c(int size, const void *src, void *dest,
-                     int model) asm("__atomic_load");
+void __atomic_load_c(int size, void *src, void *dest,
+                     int model) EXTERNAL_NAME(__atomic_load);
 
 uint8_t __atomic_load_1(uint8_t *src, int model);
 uint16_t __atomic_load_2(uint16_t *src, int model);
 uint32_t __atomic_load_4(uint32_t *src, int model);
 uint64_t __atomic_load_8(uint64_t *src, int model);
 
 void __atomic_store_c(int size, void *dest, const void *src,
-                      int model) asm("__atomic_store");
+                      int model) EXTERNAL_NAME(__atomic_store);
 
 void __atomic_store_1(uint8_t *dest, uint8_t val, int model);
 void __atomic_store_2(uint16_t *dest, uint16_t val, int model);
 void __atomic_store_4(uint32_t *dest, uint32_t val, int model);
 void __atomic_store_8(uint64_t *dest, uint64_t val, int model);
 
 void __atomic_exchange_c(int size, void *ptr, const void *val, void *old,
-                         int model) asm("__atomic_exchange");
+                         int model) EXTERNAL_NAME(__atomic_exchange);
 
 uint8_t __atomic_exchange_1(uint8_t *dest, uint8_t val, int model);
 uint16_t __atomic_exchange_2(uint16_t *dest, uint16_t val, int model);
 uint32_t __atomic_exchange_4(uint32_t *dest, uint32_t val, int model);
 uint64_t __atomic_exchange_8(uint64_t *dest, uint64_t val, int model);
 
 int __atomic_compare_exchange_c(int size, void *ptr, void *expected,
                                 const void *desired, int success, int failure)
-                                asm("__atomic_compare_exchange");
+    EXTERNAL_NAME(__atomic_compare_exchange);
 
 bool __atomic_compare_exchange_1(uint8_t *ptr, uint8_t *expected,
                                  uint8_t desired, int success, int failure);
 bool __atomic_compare_exchange_2(uint16_t *ptr, uint16_t *expected,
                                  uint16_t desired, int success, int failure);
 bool __atomic_compare_exchange_4(uint32_t *ptr, uint32_t *expected,
                                  uint32_t desired, int success, int failure);
 bool __atomic_compare_exchange_8(uint64_t *ptr, uint64_t *expected,