GCN: Tag '-march=[...]', '-mtune=[...]' as 'Negative' of themselves [PR112669]

GCN: Tag '-march=[...]', '-mtune=[...]' as 'Negative' of themselves [PR112669]

Certain other command-line flags are mutually exclusive (random example: GCN
'-march=gfx906', '-march=gfx908').  If they're not appropriately marked up,
this does disturb the multilib selection machinery, for example:

    $ build-gcc-offload-amdgcn-amdhsa/gcc/xgcc -print-multi-directory -march=gfx906
    gfx906
    $ build-gcc-offload-amdgcn-amdhsa/gcc/xgcc -print-multi-directory -march=gfx908
    gfx908
    $ build-gcc-offload-amdgcn-amdhsa/gcc/xgcc -print-multi-directory -march=gfx906 -march=gfx908
    .

In the last invocation, '-march=gfx900 -march=gfx906', for example, in
'gcc/gcc.cc:set_multilib_dir' we see both flags -- which there doesn't exist a
matching multilib for, therefore we "fail" to the default ('.').  Tagges as
'Negative', only the last flag survives, and we, for example, get the expected:

    $ build-gcc-offload-amdgcn-amdhsa/gcc/xgcc -print-multi-directory -march=gfx906 -march=gfx908
    gfx908

I quickly found that the same also applies to GCN's '-mtune=[...]', but I've
not otherwise reviewed the GCN options.

	PR target/112669
	gcc/
	* config/gcn/gcn.opt (march=, mtune=): Tag as 'Negative' of
	themselves.