[AMDGPU] Corrected directive to use for ELF weak refs

[dp]

Summary

WeakRefDirective should specify a directive to declare "a global as being a weak undefined symbol".

The directive used by AMDGPU is incorrect - ".weakref" is intended for other purposes.
The correct directive is ".weak" and it is already defined as default for ELF.

This issue was found when compiling a HIP program using -O0 -save-temps. The following declaration

%struct.__hip_builtin_blockIdx_t = type { i8 }
@blockIdx = extern_weak dso_local protected addrspace(1) global %struct.__hip_builtin_blockIdx_t, align 1

resulted in code which cannot be assembled:

.protected	blockIdx
.weakref	blockIdx

[arsenm]

Needs test

[dp]

Added a test

llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUMCAsmInfo.cpp

AMDGPUMCAsmInfo::AMDGPUMCAsmInfo(const Triple &TT,
   //===--- Data Emission Directives -------------------------------------===//
   SunStyleELFSectionSwitchSyntax = true;
   UsesELFSectionDirectiveForBSS = true;
 
   //===--- Global Variable Emission Directives --------------------------===//
   HasAggressiveSymbolFolding = true;
   COMMDirectiveAlignmentIsInBytes = false;
   HasNoDeadStrip = true;
-  WeakRefDirective = ".weakref\t";
   //===--- Dwarf Emission Directives -----------------------------------===//
   SupportsDebugInformation = true;
   DwarfRegNumForCFI = true;
 
   UseIntegratedAssembler = false;
 }
 
 bool AMDGPUMCAsmInfo::shouldOmitSectionDirective(StringRef SectionName) const {