gas: add visibility support using GNU syntax on XCOFF

In order to ease port of GNU assembly code and especially ld testsuite,
this patch allows XCOFF to accept the usual GNU syntax for visibility.

PR 22085

gas/ChangeLog:

	* config/tc-ppc.c (ppc_GNU_visibility): New function.
	* testsuite/gas/ppc/aix.exp: Add new tests.
	* testsuite/gas/ppc/xcoff-visibility-2-32.d: New test.
	* testsuite/gas/ppc/xcoff-visibility-2-64.d: New test.
	* testsuite/gas/ppc/xcoff-visibility-2.s: New test.