lang/rust: Update to 1.37.0

• Force rebuild all consumers to catch regressions early

Thanks to Mikaël Urankar for providing updated bootstraps for
aarch64, armv6, armv7, powerpc64.

Changes: https://blog.rust-lang.org/2019/08/15/Rust-1.37.0.html
Tested by: jbeich, tobik, greg@unrelenting.technology (aarch64)
With hat: rust
Differential Revision: https://reviews.freebsd.org/D21247