Motor OS was added as a no-std Tier-3 target in
https://github.com/rust-lang/rust/pull/146848
as x86_64-unknown-motor.
This patch/PR adds the std library for Motor OS.
While the patch may seem large, all it does is proxy
std pal calls to moto-rt. When there is some non-trivial
code (e.g. thread::spawn), it is quite similar, and often
identical, to what other platforms do.