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/gcc: aarch64 and sparc64.

• poudriere bulk -t freebsd-gcc6@all
• make check-plist of base/gcc6 for all archs but sparc64 sparc64 can't build a sysroot, so I can't build this port for sparc64