[ping**2] Handle MULTILIB_REUSE in auto-generated SYSROOT_SUFFIX_SPEC macro

[Sandra Loosemore]

Re-pinging a patch from last year that never got reviewed:
https://gcc.gnu.org/ml/gcc-patches/2014-06/msg00511.html

This problem still exists in GCC 5.1 and the above patch still fixes it. 
 I haven't tried mainline head yet, but it doesn't look like anything 
else has touched this since we branched GCC 5.

-Sandra