Make libm crate work with Motūrus OS

[lasiotus]

As discussed here:

https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/How.20to.20force.20libm.20linking.20for.20custom.20targets.3F

Rust toolchain/stdlib assumes there is a libm (math) library available, which is not the case in Motūrus OS, at least not in the way it is expected. A couple of basic math functions have been copied from Rust's libm crate into moto-runtime, but Rust's libm crate was created specitically, it seems, to serve cases like we have here. So it seems there should be a way to configure Rust toolchain to pick up libm crate.

[lasiotus]

More context: this patch: https://github.com/rust-lang/compiler-builtins/pull/248/files added libm support for wasm; we probably need to modify https://github.com/rust-lang/compiler-builtins/blob/master/src/math.rs to add Moturus support (and lib.rs above it).

[lasiotus]

This also could be relevant: https://github.com/rust-lang/compiler-builtins/blob/a81adbc0124b56cc404bd6b9ab99f35707162ffe/src/lib.rs#L60 (from this discussion: https://rust-lang.zulipchat.com/#narrow/channel/219381-t-libs/topic/libm.20.26.20builtins.20planning)