Replace the general USE_GCC=yes when building for powerpc64 by the
specific USE_GCC=5 to be future proof when GCC 6 becomes the default
(since USE_GCC=yes would then pull in lang/gcc6 which we obviously
cannot require to build gcc6 itself nor gcc6-devel). This also helps
users who play with GCC_DEFAULT in Mk/bsd.default-versions.mk.

Reported by: jhibbits
Discussed with: andreast