Out of curiosity, what kind of embedded device has only C and asm support? Usually, the compiler used for these is gcc which, for a really long time is written in C++. It shouldn't be hard to use even go on embedded now. Also Rust gets rustc_codegen_gcc[1], so you will be able to use it everywhere.
[1] https://github.com/rust-lang/rust/tree/master/compiler/rustc...