android: radeon: fix LLVMInitializeAMDGPU* functions declaration

android: radeon: 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/gallium/drivers/radeon/radeon_llvm_emit.c:121:2: error: implicit declaration of function 'LLVMInitializeAMDGPUTargetInfo' [-Werror=implicit-function-declaration]
  LLVMInitializeAMDGPUTargetInfo();
  ^
external/mesa/src/gallium/drivers/radeon/radeon_llvm_emit.c:122:2: error: implicit declaration of function 'LLVMInitializeAMDGPUTarget' [-Werror=implicit-function-declaration]
  LLVMInitializeAMDGPUTarget();
  ^
external/mesa/src/gallium/drivers/radeon/radeon_llvm_emit.c:123:2: error: implicit declaration of function 'LLVMInitializeAMDGPUTargetMC' [-Werror=implicit-function-declaration]
  LLVMInitializeAMDGPUTargetMC();
  ^
external/mesa/src/gallium/drivers/radeon/radeon_llvm_emit.c:124:2: error: implicit declaration of function 'LLVMInitializeAMDGPUAsmPrinter' [-Werror=implicit-function-declaration]
  LLVMInitializeAMDGPUAsmPrinter();
  ^

Acked-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Acked-by: Emil Velikov <emil.velikov@collabora.com>