lang/rust: Update to 1.50.0

[mikael]

Summary

Use bundled libgit/libssh, the versions in ports are too old
Update lang/rust-nightly to 2021-02-12
Update devel/racer to 2.1.41
Import upstream patch to fix build issue for Firefox and related ports (https://bugzilla.mozilla.org/show_bug.cgi?id=1684261)

XXX:
merge https://reviews.freebsd.org/D28520 or wait a little more for someone to comment / review it.
bump dependents ports before committing.
bump lang/rust-bootstrap

Test Plan

arch	lang/rust	consumers
11.4 i386	ok	ok
11.4 amd64	ok	ok
12.2 i386	ok	ok
12.2 amd64	ok	ok
14 i386	ok	ok
14 amd64	ok	ok
powerpc64le	ok	ok
powerpc64 elfv2	ok	ok
powerpc64 elfv1	ok	ok
14 armv7	ok
14 aarch64	ok

[mikael]

gecko, jbeich : Mk/bsd.gecko.mk still mentions rust 1.43.0, should I bump this along rust update or do you prefer to do it?

[jbeich]

In D28616#640547, @mikael wrote:

Mk/bsd.gecko.mk still mentions rust 1.43.0, should I bump this along rust update or do you prefer to do it?

Bump as I often forget, anyway. USE_GECKO depends on devel/rust-cbindgen where USES=cargo already requires a more recent version.