Avoid 64-bit Solaris/SPARC -gstabs link failures (PR ld/12152)

[Eric Botcazou]

> Since 64-bit -gstabs won't actually work in practice, it seems
> preferable that the tests not run or, if run, fail.

That would have been a valid reasoning a decade ago.  But we have accepted this 
for at least a decade so let's choose the easiest path and spend our time on 
more interesting things than discussing the handling of STABS on SPARC 64-bit.

-- 
Eric Botcazou