[PGO][Offload] Don't define GPU entrypoint on Darwin (#132966)

This PR partially reverts 83e180c and instead opts to not define the GPU entry
point on Darwin platforms. Marking `__llvm_write_custom_profile` as used
was causing issues on embedded platforms.

Author: EthanLuisMcDonough

clang/lib/Driver/ToolChains/Darwin.cpp

@@ -1481,15 +1481,11 @@ void Darwin::addProfileRTLibs(const ArgList &Args,
   // If we have a symbol export directive and we're linking in the profile
   // runtime, automatically export symbols necessary to implement some of the
   // runtime's functionality.
-  if (hasExportSymbolDirective(Args)) {
-    if (ForGCOV) {
-      addExportedSymbol(CmdArgs, "___gcov_dump");
-      addExportedSymbol(CmdArgs, "___gcov_reset");
-      addExportedSymbol(CmdArgs, "_writeout_fn_list");
-      addExportedSymbol(CmdArgs, "_reset_fn_list");
-    } else {
-      addExportedSymbol(CmdArgs, "___llvm_write_custom_profile");
-    }
+  if (hasExportSymbolDirective(Args) && ForGCOV) {
+    addExportedSymbol(CmdArgs, "___gcov_dump");
+    addExportedSymbol(CmdArgs, "___gcov_reset");
+    addExportedSymbol(CmdArgs, "_writeout_fn_list");
+    addExportedSymbol(CmdArgs, "_reset_fn_list");
   }
 
   // Align __llvm_prf_{cnts,bits,data} sections to the maximum expected page

compiler-rt/lib/profile/InstrProfilingFile.c

@@ -1273,6 +1273,7 @@ COMPILER_RT_VISIBILITY int __llvm_profile_set_file_object(FILE *File,
   return 0;
 }
 
+#ifndef __APPLE__
 int __llvm_write_custom_profile(const char *Target,
                                 const __llvm_profile_data *DataBegin,
                                 const __llvm_profile_data *DataEnd,
@@ -1381,5 +1382,6 @@ int __llvm_write_custom_profile(const char *Target,
 
   return ReturnValue;
 }
+#endif
 
 #endif