New external GCC toolchain ports/packages

In the interest of supporting newer versions of GCC for a base system
toolchain, I've renamed the external GCC packages from <arch>-gcc
to <arch>-gcc6.  These are built as flavors of a new devel/freebsd-gcc6
port.  The xtoolchain package is not used for these new packages, instead
one does 'pkg install mips-gcc6' to get the GCC 6.x MIPS compiler and
uses 'CROSS_TOOLCHAIN=mips-gcc6'.  I've also gone ahead and updated this
compiler to 6.5.0.

I will leave the old ports/packages around for now to permit an easy
transition, but going forward, the <arch>-gcc6 packages should be preferred
to <arch>-xtoolchain-gcc for all but riscv (riscv64-gcc and riscv64-xtoolchain-gcc
are separate from the powerpc64-gcc set of packages).

In addition, I've also just added a devel/freebsd-gcc9 package which
builds <arch>-gcc9 packages.  It adds powerpc and riscv flavors relative
to freebsd-gcc6 and uses GCC 9.2.0.  To date in my testing I've yet to
be able to finish a buildworld on any of the platforms I've tried
(amd64, mips, sparc64), but the packages should permit other developers
to get the tree building with GCC 9.  To use these packages one would do
something like:

# pkg install amd64-gcc9
# make buildworld CROSS_TOOLCHAIN=amd64-gcc9

You can install both the gcc6 and gcc9 versions of a package at the same
time, e.g. amd64-gcc6 and amd64-gcc9.  Having different packages for major
versions is similar to llvm and will also let us keep a known-good
toolchain package for older releases while using newer major versions on
newer FreeBSD releases (e.g gcc9 for 13.0 and gcc6 for 12.x).

I do plan to switch the default toolchains for make universe/tinderbox
for targets using <arch>-xtoolchain-gcc based on GCC 6 over to the
freebsd-gcc6 variants in the next week or so.

-- 
John Baldwin