[CMake] Fix AMDGPUTests -DBUILD_SHARED_LIBS=on builds and trim dependencies of...

[CMake] Fix AMDGPUTests -DBUILD_SHARED_LIBS=on builds and trim dependencies of AMDGPUTests and AMDDwarfTests after D76357/G24698e526f619271705fe72bcaa928be9bc82484

FAILED: unittests/Target/AMDGPU/AMDGPUTests
...
ld.lld: error: undefined symbol: llvm::MCRegisterInfo::getLLVMRegNum(unsigned int, bool) const
>>> referenced by DwarfRegMappings.cpp:60 (/usr/local/google/home/maskray/llvm/llvm/unittests/Target/AMDGPU/DwarfRegMappings.cpp:60)
>>>               unittests/Target/AMDGPU/CMakeFiles/AMDGPUTests.dir/DwarfRegMappings.cpp.o:(AMDGPUDwarfRegMappingTests_TestWave64DwarfRegMapping_Test::TestBody())
>>> referenced by DwarfRegMappings.cpp:82 (/usr/local/google/home/maskray/llvm/llvm/unittests/Target/AMDGPU/DwarfRegMappings.cpp:82)
>>>               unittests/Target/AMDGPU/CMakeFiles/AMDGPUTests.dir/DwarfRegMappings.cpp.o:(AMDGPUDwarfRegMappingTests_TestWave32DwarfRegMapping_Test::TestBody())

A -DBUILD_SHARED_LIBS=off build is good because AMDGPUCodeGen pulls in MC.
A -DBUILD_SHARED_LIBS=on build requires all direct dependencies (MC) to be listed becuase llvm/cmake/modules/HandleLLVMOptions.cmake uses -Wl,-z,defs