Rename base/gcc to base/gcc6.

This is in preparation for adding a base/gcc9 port.

While here, update to GCC 6.5.0.

Share packing lists with the freebsd-gcc6 port as much as posible.
Split most of the freebsd-gcc6 pkg-plist into a pkg-plist.common that
lists arch-independent files under lib/. Change base/gcc6 to use the
arch-specific packing lists and pkg-list.common from freebsd-gcc6. In
addition, hook up packing lists for the remaining archs supported by
freebsd-gcc6 but not base/gcc6: aarch64 and sparc64.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D22900