lang/rust: Update to 1.40.0

• 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/2019/12/19/Rust-1.40.0.html
Tested by: mikael.urankar@gmail.com, tobik
With hat: rust
Differential Revision: https://reviews.freebsd.org/D22843