mbind gas tests and supports_gnu_osabi testsuite infrastructure

mbind gas tests and supports_gnu_osabi testsuite infrastructure

With the addition of section16 tests we have multiple tests
advertising themselves as "mbind sections" and "mbind section
contents".  This patch fixes that, and fails on quite a few targets
that force an OSABI value.  It's a pain specifying all the relevant
arm targets on an xfail line, so I wrote supports_gnu_osabi.

binutils/
	* testsuite/lib/binutils-common.exp (match_target): Accept '!' before
	TCL procedure.
	(supports_gnu_osabi): New procedure.
	(is_generic): New, from ld-lib.exp.
	(supports_gnu_unique): Use the above.
gas/
	* testsuite/gas/elf/section12a.d: Use supports_gnu_osabi in
	xfail, and rename test.
	* testsuite/gas/elf/section12b.d: Likewise.
	* testsuite/gas/elf/section16a.d: Likewise.
	* testsuite/gas/elf/section16b.d: Likewise.
ld/
	* testsuite/lib/ld-lib.exp (is_generic): Delete.
	* testsuite/ld-unique/unique.exp: Exclude tic6x.