Fix failure on powerpc 32-bit only targets

Fix failure on powerpc 32-bit only targets

Targets that lack ppc64 support were failing the new prefix-reloc
test.  This patch adds some test infrastructure to deal with that, and
changes the powerpc gas usage info so that "-a64" is omitted when
unsupported.

I've been meaning to break up the usage message for a long time;
While doing so causes translators some work now, it should make it
easier next time a new powerpc option is added.

	* config/tc-ppc.c (is_ppc64_target): New function.
	(md_show_usage): Split up usage message.  Don't show -a64 when
	unsupported.
	testsuite/gas/ppc/ppc.exp (supports_ppc64): New.
	(prefix-reloc): Only run for ppc64.