android: amd/common: fix LLVMInitializeAMDGPU* functions declaration

android: amd/common: fix LLVMInitializeAMDGPU* functions declaration

LLVMInitializeAMDGPU* functions need to be explicitly declared
and mesa expects them via <llvm-c/Target.h> header,
but LLVM needs to be instructed to invoke its own LLVM_TARGET(AMDGPU) macro,
or the functions will not be available.

A new llvm cflag (-DFORCE_BUILD_AMDGPU) serves this purpose,
the same mechanism is used also by other llvm targets e.g. FORCE_BUILD_ARM

A necessary prerequisite is to have AMDGPU target handled accordingly
in llvm config files i.e. {Target,AsmParser,AsmPrinter}.def
for llvm device build includes.

This avoids the following building errors:

external/mesa/src/amd/common/ac_llvm_util.c:43:2: error: implicit declaration of function 'LLVMInitializeAMDGPUTargetInfo' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
        LLVMInitializeAMDGPUTargetInfo();
        ^
external/mesa/src/amd/common/ac_llvm_util.c:44:2: error: implicit declaration of function 'LLVMInitializeAMDGPUTarget' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
        LLVMInitializeAMDGPUTarget();
        ^
external/mesa/src/amd/common/ac_llvm_util.c:45:2: error: implicit declaration of function 'LLVMInitializeAMDGPUTargetMC' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
        LLVMInitializeAMDGPUTargetMC();
        ^
external/mesa/src/amd/common/ac_llvm_util.c:46:2: error: implicit declaration of function 'LLVMInitializeAMDGPUAsmPrinter' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
        LLVMInitializeAMDGPUAsmPrinter();
        ^
Acked-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Acked-by: Emil Velikov <emil.velikov@collabora.com>