[profile] Make {__start_,__stop_}__llvm_prf_* symbols undefined weak

To make a kind of metadata section usage work, we want to drop the
`__start_/__stop_ references retain C identifier name sections` rule from LLD (see D96914).

If an application has no `__llvm_prf_data` input section surviving --gc-sections,
LLD will error for undefined hidden `{__start_,__stop_}__llvm_prf_*` from `libclang_rt.profile-*`.
Other `__llvm_prf_*` sections have similar issues.

Making the references weak can address the problem.
This probably enables the opportunity to drop zero size dummy sections in `InstrProfilingPlatformLinux.c`.

Reviewed By: davidxl

Differential Revision: https://reviews.llvm.org/D96936

Author: MaskRay

compiler-rt/lib/profile/InstrProfilingPlatformLinux.c

@@ -26,15 +26,17 @@
 /* Declare section start and stop symbols for various sections
  * generated by compiler instrumentation.
  */
-extern __llvm_profile_data PROF_DATA_START COMPILER_RT_VISIBILITY;
-extern __llvm_profile_data PROF_DATA_STOP COMPILER_RT_VISIBILITY;
-extern uint64_t PROF_CNTS_START COMPILER_RT_VISIBILITY;
-extern uint64_t PROF_CNTS_STOP COMPILER_RT_VISIBILITY;
-extern uint32_t PROF_ORDERFILE_START COMPILER_RT_VISIBILITY;
-extern char PROF_NAME_START COMPILER_RT_VISIBILITY;
-extern char PROF_NAME_STOP COMPILER_RT_VISIBILITY;
-extern ValueProfNode PROF_VNODES_START COMPILER_RT_VISIBILITY;
-extern ValueProfNode PROF_VNODES_STOP COMPILER_RT_VISIBILITY;
+extern __llvm_profile_data PROF_DATA_START COMPILER_RT_VISIBILITY
+    COMPILER_RT_WEAK;
+extern __llvm_profile_data PROF_DATA_STOP COMPILER_RT_VISIBILITY
+    COMPILER_RT_WEAK;
+extern uint64_t PROF_CNTS_START COMPILER_RT_VISIBILITY COMPILER_RT_WEAK;
+extern uint64_t PROF_CNTS_STOP COMPILER_RT_VISIBILITY COMPILER_RT_WEAK;
+extern uint32_t PROF_ORDERFILE_START COMPILER_RT_VISIBILITY COMPILER_RT_WEAK;
+extern char PROF_NAME_START COMPILER_RT_VISIBILITY COMPILER_RT_WEAK;
+extern char PROF_NAME_STOP COMPILER_RT_VISIBILITY COMPILER_RT_WEAK;
+extern ValueProfNode PROF_VNODES_START COMPILER_RT_VISIBILITY COMPILER_RT_WEAK;
+extern ValueProfNode PROF_VNODES_STOP COMPILER_RT_VISIBILITY COMPILER_RT_WEAK;
 
 /* Add dummy data to ensure the section is always created. Add used attribute so
  * that they are linker GC roots on supported ELF platforms.