Enable parallel building for GCC.

Even if using profiledbootstrap.  This was unsafe before 4.8, and
then the documentation was not fixed on time.

The documentation got fixed here:
  https://github.com/gcc-mirror/gcc/commit/c763997f340ec1fab37ad538b57afdad4f4bf747

But the actual code was already fixed here:
  https://github.com/gcc-mirror/gcc/commit/5d2fca09d543d4b42b99fe20f412efb78cc50ec3

So this is safe both for GCC 4.8 and GCC 4.9.