amdgcn: Enable TImode

amdgcn: Enable TImode

This enables types __int128 et al for move, add, subtract, and logical
operations.  At least shift, rotate, multiple, divide, and modulus are broken
so we can expect some test failures.

This is required now because libgomp no longer builds without __int128.

An additional patch will be required to unbreak the libgfortran build.

gcc/ChangeLog:

	* config/gcn/gcn.c (gcn_scalar_mode_supported_p): New function.
	(TARGET_SCALAR_MODE_SUPPORTED_P): New define.