mrustc: init at 0.9

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.

pkgs/development/compilers/mrustc/patches/0001-use-shared-llvm.patch

@@ -0,0 +1,12 @@
+--- a/rustc-1.29.0-src/src/librustc_llvm/lib.rs
+--- b/rustc-1.29.0-src/src/librustc_llvm/lib.rs
+@@ -23,6 +23,9 @@
+ #![feature(link_args)]
+ #![feature(static_nobundle)]
+ 
++// https://github.com/rust-lang/rust/issues/34486
++#[link(name = "ffi")] extern {}
++
+ // See librustc_cratesio_shim/Cargo.toml for a comment explaining this.
+ #[allow(unused_extern_crates)]
+ extern crate rustc_cratesio_shim;