This is basically LLVMTargetMachineEmitToMemoryBuffer inlined and reworked.

struct ac_compiler_passes (opaque type) contains the main pass manager.

ac_create_llvm_passes -- the result can go to thread local storage
ac_destroy_llvm_passes -- can be called by a destructor in TLS
ac_compile_module_to_binary -- from LLVMModuleRef to ac_shader_binary

The motivation is to do the expensive call addPassesToEmitFile once
per context or thread.

Reviewed-by: Dave Airlie <airlied@redhat.com>