gas: add visibility support using GNU syntax on XCOFF

[Alan Modra]

On Thu, Jan 13, 2022 at 08:07:26AM +0000, CHIGOT, CLEMENT wrote:
> Hi Alan,
> 
> Yes both are fine.
> However, how are you building binutils on AIX ? By default,
> reading NULL is possible on AIX that's why it didn't trigger any
> segfault in my build.

I generally build using a powerpc64le-linux or x86_64-linux host,
so I'm testing cross-builds.  Incidentally, I see
rs6000-aix4.3.3  +FAIL: XCOFF Visibility 1 (32 bit)
rs6000-aix4.3.3  +FAIL: XCOFF Visibility 1 (64 bit)

(That was --build=x86_64-linux --target=rs6000-aix4.3.3)

-- 
Alan Modra
Australia Development Lab, IBM