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/0003-echo-newlines.patch

@@ -0,0 +1,13 @@
+--- a/run_rustc/Makefile
++++ b/run_rustc/Makefile
+@@ -103,7 +103,9 @@
+ else
+ 	cp $(OUTDIR)build-rustc/release/rustc_binary $(BINDIR)rustc_binary
+ endif
+-	echo '#!/bin/sh\nd=$$(dirname $$0)\nLD_LIBRARY_PATH="$(abspath $(LIBDIR))" $$d/rustc_binary $$@' >$@
++	echo '#!$(shell which bash)' > $@
++	echo 'd=$$(dirname $$0)' >> $@
++	echo 'LD_LIBRARY_PATH="$(abspath $(LIBDIR))" $$d/rustc_binary $$@' >> $@
+ 	chmod +x $@
+ 
+ $(BINDIR)hello_world: $(RUST_SRC)test/run-pass/hello.rs $(LIBDIR)libstd.rlib $(BINDIR)rustc