[AMDGPU] Fix "use of uninitialized variable" static analyzer warning. NFCI.

Add "unreachable" default case to AMDGPUTargetStreamer::getArchNameFromElfMach

Author: RKSimon

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

@@ -60,6 +60,7 @@ StringRef AMDGPUTargetStreamer::getArchNameFromElfMach(unsigned ElfMach) {
   AMDGPU::GPUKind AK;
 
   switch (ElfMach) {
+  default: llvm_unreachable("Unhandled ELF::EF_AMDGPU type");
   case ELF::EF_AMDGPU_MACH_R600_R600:      AK = GK_R600;    break;
   case ELF::EF_AMDGPU_MACH_R600_R630:      AK = GK_R630;    break;
   case ELF::EF_AMDGPU_MACH_R600_RS880:     AK = GK_RS880;   break;