[build] Only support -gstabs on Mac OS X if assember supports it (PR target/67973)

[Mike Stump]

On Dec 16, 2015, at 11:29 PM, Rainer Orth <ro@CeBiTec.Uni-Bielefeld.DE> wrote:
> Here's what I came up with.  Tested with the appropriate runtest
> invocations both in a tree with the Xcode 7/LLVM as without stabs
> support, where the tests come out UNSUPPORTED, and another one with the
> Xcode 6.4/gas as with stabs, where they PASS.
> 
> I've left alone two testcases using -gstabs* which are guaranteed to
> work without the keyword:
> 
> 	gcc.target/powerpc/stabs-attrib-vect-darwin.c
> 	gcc.target/s390/20041216-1.c	
> 
> In case the current test for stabs (checking if one can compile/assemble
> with -gstabs) isn't enough on some of the targets currently listed
> explicitly, it could easily be augmented.
> 
> Ok for mainline?

Ok.