And switch from Rust 1.29 to 1.54, as this is the top version supported by mrustc
Probably after gcc update. Anyway, including standard headers should be safe even if not utilized (and they do seem here).
mrustc is mostly patched to use shared LLVM sources but still uses in-tree source for compiler-rt from LLVM 7. This needs to be patched to compile under glibc 2.31 or later. It's easy enough to reapply all our compiler-rt patches here.
