ld: add hidden and internal visibility support for XCOFF

This patch adds a primary support for hidden and internal visibility in
GNU linker for XCOFF format.
The protected visibility isn't yet supported.

PR 22085

bfd/ChangeLog:

	* xcofflink.c (xcoff_dynamic_definition_p): Add hidden
	  and internal visibility support.
	(xcoff_link_add_symbols): Likewise.
	(xcoff_auto_export_p): Likewise.
	(bfd_xcoff_export_symbol): Likewise.
	(xcoff_link_input_bfd): Likewise.

ld/ChangeLog:

	* testsuite/ld-vsb/main.c: Adapt for XCOFF.
	* testsuite/ld-vsb/sh1.c: Likewse.
	* testsuite/ld-vsb/vsb.exp: Likewise.
	* testsuite/ld-vsb/visibility-1-xcoff-32.d: New test.
	* testsuite/ld-vsb/visibility-1-xcoff-64.d: New test.
	* testsuite/ld-vsb/visibility-2-xcoff-32.d: New test.
	* testsuite/ld-vsb/visibility-2-xcoff-64.d: New test.
	* testsuite/ld-vsb/xcoffvsb.dat: New test.