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

[Rainer Orth]

Hi Iain,

>>> Hrm, this needs more investigation, and will affect 10.10 too, since xc7 is
>>> the default there.
>>> (separate issue, let’s start a new PR, or at least a new thread).
>> 
>> right, but it's only an issue if you switch assemblers (or linkers) used
>> by gcc without rebuilding.  This has never been safe on any platform.
>
> The issue that worries me is that the new assembler supports .cfi_xxx
> (YAY!), but the Darwin port is not 100% ready for it yet (BOO!) (I have
> patches, and expect to make them available for folks to try in the next ~ 2
> weeks).  However, still not sure that they would exactly be stage3 stuff.

ah, I see.  The platform maintainers have some leeway even in stage 3,
so let's see when you post the patches.

> Did you say that bootstrap fails if -Q is jammed in everywhere?
> (that would be a short-term safety net, falling back to the cctools assembler).

Yes, but only if you run the configure test with the LLVM assembler, but
always call as -Q during the build.  It should be possible to perform
the assembler tests with as -Q instead, so there shouldn't be an
inconsistency and you get the same result as if configuring with a copy
of the Xcode 6.4 as.

	Rainer

-- 
-----------------------------------------------------------------------------
Rainer Orth, Center for Biotechnology, Bielefeld University