android: conditionally add LLVM_TARGET(AMDGPU) in device configs

android: conditionally add LLVM_TARGET(AMDGPU) in device configs

Adding LLVM_TARGET(AMDGPU) causes LLVMInitializeAMDPU* prototypes definition,
but this needs to be controlled by defining FORCE_BUILD_AMDGPU,
in order to avoid building errors in clients not having AMDGPU support
like for example md.lc which is broken by LLVM_TARGET(AMDGPU)

Mesa amd/common and gallium radeonsi will require to define FORCE_BUILD_AMDGPU
in Android makefiles in order to materialize LLVMInitializeAMDGPU* prototypes