gas: add visibility support for XCOFF

gas: add visibility support for XCOFF

XCOFF assembly defines the visibility using an additional argument
on several pseudo-ops: .globl, .weak, .extern and .comm.
This implies that .globl and .weak syntax is different than the
usual GNU syntax. But we want to provide compatibility with AIX
assembler, especially because GCC is generating the visibility
using this XCOFF syntax.

PR 22085

bfd/ChangeLog:

        * coffcode.h (coff_write_object_contents): Change XCOFF header
        vstamp field to 2.
        * coffgen.c (coff_print_symbol): Increase the size for n_type.

gas/ChangeLog:

        * config/tc-ppc.c (ppc_xcoff_get_visibility): New function.
        (ppc_globl): New function.
        (ppc_weak): New function.
        (ppc_comm): Add visibility field support.
        (ppc_extern): Likewise.
        * testsuite/gas/all/cofftag.d: Adjust to new n_type size
        providing by objdump.
        * testsuite/gas/ppc/test1xcoff32.d: Likewise.
        * testsuite/gas/ppc/aix.exp: Add new tests.
        * testsuite/gas/ppc/xcoff-visibility-1-32.d: New test.
        * testsuite/gas/ppc/xcoff-visibility-1-64.d: New test.
        * testsuite/gas/ppc/xcoff-visibility-1.s: New test.

include/ChangeLog:

        * coff/internal.h (SYM_V_INTERNAL, SYM_V_HIDDEN,
        SYM_V_PROTECTED, SYM_V_EXPORTED, SYM_V_MASK): New defines.
        * coff/xcoff.h (struct xcoff_link_hash_entry): Add visibility
        field.

ld/ChangeLog:

        * testsuite/ld-pe/pr19803.d: Adjust to new n_type size
        providing by objdump.