Log Message:
Upgrade rust to version 1.31.1.

Pkgsrc changes:
 * Sadly, I had to reinstate the "make tar files" rust code to make
   it possible to build cross-compiled bootstrap kits.
 * Add an adjustable "BUILD_TARGET", "dist" for cross-building
   a bootstrap kit, "build" for a normal native build.
 * New bootstrap kits built for NetBSD/powerpc, NetBSD/earmv7hf,
   and NetBSD/sparc64 version 1.31.1.
 * gcc-wrap script amended to also drop -Wl,--enable-new-dtags
   (so it could be used outside pkgsrc)
 * Worked around use of AtomicU64 in release build tool (ugly band-aid patch).
   Some platforms lack support for that type and associated operations.

Upstream changes:
 - [Fix Rust failing to build on `powerpc-unknown-netbsd`][56562]
 - [Fix broken go-to-definition in RLS][rls/1171]
 - [Fix infinite loop on hover in RLS][rls/1170]

[56562]: https://github.com/rust-lang/rust/pull/56562
[rls/1171]: https://github.com/rust-lang/rls/issues/1171
[rls/1170]: https://github.com/rust-lang/rls/pull/1170