Support Sun as visibility syntax

[Eric Botcazou]

> I wouldn't call this conventional wisdom, but just another case of
> inconsistencies between ports.  E.g. i?86-*-solaris2* uses tm_defines
> for TARGET_GNU_LD=1 and TARGET_BI_ARCH=1, m68k*-*-openbsd* for
> OBSD_OLD_GAS, mips-sgi-irix[56]* for IRIX_USING_GNU_LD, and many more.
> I'm just trying to get away from the huge number of single-line headers
> which can just as well be handled by tm_defines.

Do whatever you like with other macros but, again, not for USE_GAS.  There is 
at least one thing consistent in config.gcc, it's that no ports define it.
Since you aren't maintainer of all the ports, defining it for Solaris will 
introduce an inconsistency where there is none.

-- 
Eric Botcazou