Building lang/rust on Linux with Musl libc

[Nathan Fisher <nfisher.sr%gmail.com@localhost>]

This port needs to fetch a binary toolchain to bootstrap the compiler 
from source. It fails on Linux using Musl libc because after checking 
what the OS is and determining that it is Linux it automatically grabs 
the "gnu" version of the toolchain.

.if ${MACHINE_PLATFORM:MLinux-*-aarch64} || make(distinfo) || make 
(makesum) || make(mdi)
RUST_STAGE0_VER=    1.68.2
RUST_ARCH:=        aarch64-unknown-linux-gnu
...etc

I'm aware that there is no particularly good way to check for Musl at 
compile time, but it would be great if this port could at the very least 
have a way to override the assumption of Glibc, even if it's just 
manually using the package options framework. Probably better would be 
if there were a global flag that could go into /usr/pkg/etc/mk.conf as I 
assume there have to be other packages that would benefit by knowing 
that they are being compiled on a Musl system? I'd be happy to put some 
work into a fix if there was some concensus as to what the best way 
forward is.

Thanks -

Nate