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)