Patch for bug 25524 (-imultilib)

[Benjamin Kosnik]

> Bootstrapped with no regressions on x86_64-unknown-linux-gnu.  OK to
> commit?  (The header differences on this platform are in the definitions 
> of _GLIBCXX_PTRDIFF_T_IS_INT and _GLIBCXX_SIZE_T_IS_UINT, which vary 
> between the 32-bit and 64-bit multilibs, and of course in the precompiled 
> headers.)

The libstdc++ bits look fine to me. 

Thanks for fixing this.

-benjamin