amdgcn: Silence warnings in gcn.c

amdgcn: Silence warnings in gcn.c

This fixes a few cases of "unquoted identifier or keyword", one "spurious
trailing punctuation sequence", and a "may be used uninitialized".

gcc/ChangeLog:

	* config/gcn/gcn.c (gcn_parse_amdgpu_hsa_kernel_attribute): Add %< and
	  %> quote markers to error messages.
	(gcn_goacc_validate_dims): Likewise.
	(gcn_conditional_register_usage): Remove exclaimation mark from error
	message.
	(gcn_vectorize_vec_perm_const): Ensure perm is fully uninitialized.