compiler-rt is C code and we don't bootstrap a C compiler, so it can be built once in the same way as [libbacktrace and jemalloc](https://github.com/rust-lang/rust/pull/39329). This issue may be obsoleted by ["rewrite compiler-rt in Rust"](https://github.com/rust-lang/rust/issues/35437) project.