devel/freebsd-gcc12: External FreeBSD toolchain based on GCC 12.

This port provides external GCC toolchains for GCC 12.1.0 for the
aarch64, amd64, armv6, armv7, i386, powerpc, powerpc64, and riscv64
platforms.

Reviewed by: lwhsu
Differential Revision: https://reviews.freebsd.org/D35685