amdgcn: Support LLVM 13 assembler syntax

The LLVM devs have changed the assembler architecture attribute names on both
CLI and in the ".amdgcn_target" directive, and changed the attribute syntax
inside the directive, without keeping any backwards compatibility. :-(

This patch improves our configure tests to detect what dialect to use, what
attributes are valid, and adjusts the specs to match.

gcc/ChangeLog:

	* config.in: Regenerate.
	* config/gcn/gcn-hsa.h (X_FIJI): New macro.
	(X_900): New macro.
	(X_906): New macro.
	(X_908): New macro.
	(A_FIJI): Rename to ...
	(S_FIJI): ... this.
	(A_900): Rename to ...
	(S_900): ... this.
	(A_906): Rename to ...
	(S_906): ... this.
	(A_908): Rename to ...
	(S_908): ... this.
	(SRAMOPT): New macro.
	(ASM_SPEC): Adjust xnack option usage.
	* config/gcn/gcn.c (output_file_start): Adjust amdgcn_target usage.
	* configure: Regenerate.
	* configure.ac: Detect LLVM assembler dialect.