attribs: Cache the gnu namespace

Later patches add more calls to get_attribute_namespace.
For scoped attributes, this is a simple operation on tree pointers.
But for normal GNU attributes (the vast majority), it involves a
call to get_identifier ("gnu").  This patch caches the identifier
for speed.

gcc/
	* Makefile.in (GTFILES): Add attribs.cc.
	* attribs.cc (gnu_namespace_cache): New variable.
	(get_gnu_namespace): New function.
	(lookup_attribute_spec): Use it instead of get_identifier ("gnu").
	(get_attribute_namespace, attribs_cc_tests): Likewise.