amdgcn: Fix attributes for LLVM-12 [PR 100208]

amdgcn: Fix attributes for LLVM-12 [PR 100208]

This should work for a wider range of LLVM 12 variants now.
More work required for LLVM 13 though.

gcc/ChangeLog:

	PR target/100208
	* config.in: Regenerate.
	* config/gcn/gcn-hsa.h (A_FIJI): New define.
	(A_900): New define.
	(A_906): New define.
	(A_908): New define.
	(ASM_SPEC): Use A_FIJI, A_900, A_906 and A_908.
	* config/gcn/gcn.c (output_file_start): Adjust attributes according
	to the assembler capabilities.
	* config/gcn/mkoffload.c (main): Likewise.
	* configure: Regenerate.
	* configure.ac: Add tests for LLVM assembler attribute features.