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/0002-dont-build-llvm.patch

@@ -0,0 +1,14 @@
+--- a/minicargo.mk
++++ b/minicargo.mk
+@@ -116,11 +116,6 @@
+ LLVM_CMAKE_OPTS += CMAKE_BUILD_TYPE=RelWithDebInfo
+ 
+ 
+-$(LLVM_CONFIG): $(RUSTCSRC)build/Makefile
+-	$Vcd $(RUSTCSRC)build && $(MAKE)
+-$(RUSTCSRC)build/Makefile: $(RUSTCSRC)src/llvm/CMakeLists.txt
+-	@mkdir -p $(RUSTCSRC)build
+-	$Vcd $(RUSTCSRC)build && cmake $(addprefix -D , $(LLVM_CMAKE_OPTS)) ../src/llvm
+ 
+ 
+ #