PR19525: Build testing of non-default multilibs

[Zack Weinberg]

I tested this patch on a wide variety of systems:

  hppa2.0w-hp-hpux11.11  } no multilibs
  sparc-sun-solaris2.8   }

  ia64-hp-hpux11.23           }
  powerpc64-unknown-linux-gnu } 32bit/64bit multilibs
  x86_64-unknown-linux-gnu    }

In all cases, bootstrap completed successfully, and I see no
regressions for the default multilib.  

My test scripts only ran dejagnu against the non-default multilib on
ia64-hp-hpux11.23; this is an oversight on my end.  On that system,
however, results are very bad.  With -mlp64 (which triggers the
non-default multilib) all link and execute tests against the shared
libgcc fail.  The error is

ld: Can't find library or mismatched ABI for -lgcc_s_hpux64
Fatal error.
collect2: ld returned 1 exit status

This is because "-lgcc_s_hpux64" was hardwired into specs in
ia64/hpux.h.  I am retesting with the definition of LIBGCC_SPEC
removed from that file.  I'm also repeating the powerpc64-linux and
x86_64-linux tests with dejagnu instructed to cycle through all
multilibs.

zw