Build all mobile Rust targets in a single compilation step

While we can't use more than one --target argument we can comma-separate the Rust targets we want to build. Thus, no need to compile five times e.g LLVM just to build Rust for all platforms.

(Note: In principle this works for desktop, too, but the different toolchains needed for the different platforms makes this (slightly) more complicated. So, we start with mobile-only here first)