armv6 support for rust?

[nia <nia%NetBSD.org@localhost>]

Right now we only build rust for armv7, but the RPI Zero and
other cheapo Rapsberry Pi boards are v6. Since rust is becoming
even more of a critical dependency (compare py-cryptography),
it would be really nice to get rust for armv6.

It can't be the only 32-bit arm port (like how we downgraded
the 32-bit x86 one to i586 to support embedded boards), because
backwards compatibility is less of a priority for the arm folks and
newer hardware is missing deprecated instructions.

It looks like there is already support upstream, so it just
falls on there being existing bootstrap tarballs, hopefully? :)

Nia