Update base/gcc to work with devel/freebsd-gcc6.

• Explicitly set all the toolchain <foo>_FOR_TARGET variables. GCC's default guess for CC, etc. happened to work with the powerpc64-gcc port, but fails when the binaries have a suffix like the '6' suffix in freebsd-gcc6. Setting these variables explicitly keeps GCC's build from having to guess.
• Pull patches from freebsd6-gcc instead of powerpc64-gcc.
• Use per-arch plists.
• Simplify the post-install steps to keep the hard links the normal install performs and avoid renaming binaries. Keep links to 'cc' and 'c++'.

Reviewed by: bapt (earlier version)
Differential Revision: https://reviews.freebsd.org/D22594