mrustc: init

This contains some extraordinarily ugly hacks for a set of even uglier
Makefiles.

Signed-off-by: Ophestra <cat@gensokyo.uk>

mrustc/0001-do-not-download-rustc.patch

@@ -0,0 +1,27 @@
+From 157772049aa7da13533026292cb4f67c1e8bf542 Mon Sep 17 00:00:00 2001
+From: nobody <nobody@localhost>
+Date: Sun, 19 Jul 2026 06:21:49 +0900
+Subject: [PATCH 1/3] do not download rustc
+
+The $(RUSTC_SRC_DL) file already exists, but for some reason Make wants
+to rebuild this target when it has $(RUSTC_SRC_TARBALL) as a dependency.
+---
+ minicargo.mk | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/minicargo.mk b/minicargo.mk
+index 5cd3489c..eb869a1b 100644
+--- a/minicargo.mk
++++ b/minicargo.mk
+@@ -233,7 +233,7 @@ $(RUSTC_SRC_DL): rustc-$(RUSTC_VERSION)-src/extracted rustc-$(RUSTC_VERSION)-src
+ # - libstd, libpanic_unwind, libtest and libgetopts
+ # - libproc_macro (mrustc)
+ ifeq ($(USE_MERGED_BUILD),1)
+-$(RUSTCSRC)mrustc-stdlib/Cargo.toml: $(RUSTC_SRC_DL) minicargo.mk
++$(RUSTCSRC)mrustc-stdlib/Cargo.toml: minicargo.mk
+ 	@mkdir -p $(dir $@)
+ 	@echo "#![no_core]" > $(dir $@)/lib.rs
+ 	@echo "[package]" > $@
+-- 
+2.55.0
+