devel/freebsd-gcc15: External FreeBSD toolchain based on GCC 15

This port provides external GCC toolchains for GCC 15.2.0 for the
aarch64, amd64, armv7, i386, powerpc, powerpc64, powerpc64le, and
riscv64 platforms.

Reviewed by: lwhsu
Differential Revision: https://reviews.freebsd.org/D54834