selfhost-verify: parchea el hello_world final de run_rustc (snag #2 en Makefile:262)

Con el muro de proc-macros roto (stage0 musl-host), run_rustc avanzó por primera vez
hasta el smoke-test hello_world del prefix FINAL (Makefile:262) y falló igual que el
de stage-1: usa el default musl -static-pie + CRT self-contained ausente
(rcrt1.o/crti.o/crtbeginS.o/crtendS.o/crtn.o + -lunwind estático) -> ld: cannot find.

Fix idéntico al de la línea 172: añade --target $(RUSTC_TARGET) -C target-feature=
-crt-static -C link-self-contained=no a la regla del hello final. run_rustc.patch
regenerado (vs be69c74) e incluye ahora ambas reglas; README snag #2 documenta las dos.

Verificado parcialmente: stage-2 rustc, stage-3 rustc y cargo se construyeron OK antes
del fallo (proc-macros nativos: tinystr/displaydoc/icu compilan). Rebuild en curso.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/rust-frontier/README.md

@@ -40,9 +40,12 @@ to bootstrap a usable rust there:
 
 2. **self-contained CRT absent.** musl's default link is `-static-pie` against a
    `lib/rustlib/<t>/lib/self-contained/` CRT dir that the mrustc sysroot never
-   populates. Fix: `-C target-feature=-crt-static -C link-self-contained=no` so the
-   Alpine gcc supplies crt1/crti/crtn + crtbeginS/crtendS and links dynamically
-   against the system musl (RUSTFLAGS in Makefile:111, plus the per-link sites).
+   populates (also needs static `-lunwind`). Fix: `-C target-feature=-crt-static
+   -C link-self-contained=no` so the Alpine gcc supplies crt1/crti/crtn +
+   crtbeginS/crtendS and links dynamically against the system musl (RUSTFLAGS in
+   Makefile:111, plus **both** hello_world smoke-test rules: the stage-1 one at
+   Makefile:172 *and* the final fully-bootstrapped one at Makefile:262 — the latter
+   only surfaces once stage-2/3 actually completes, i.e. after snag #3 is fixed).
 
 3. **host-triple = gnu on a musl system** (the deep one). cargo compiles
    build-scripts and **proc-macros** for the *host*; mrustc's stage0 rustc has its

scripts/rust-frontier/run_rustc.patch

@@ -1,5 +1,5 @@
 diff --git a/run_rustc/Makefile b/run_rustc/Makefile
-index e78d8c5..4d6ef5d 100644
+index e78d8c5..00162a7 100644
 --- a/run_rustc/Makefile
 +++ b/run_rustc/Makefile
 @@ -108,7 +108,7 @@ CARGO_FLAGS := --target $(RUSTC_TARGET) -j $(PARLEVEL) --release --verbose
@@ -29,6 +29,15 @@ index e78d8c5..4d6ef5d 100644
  
  # ---
  # Stage 2: Build a proper libstd (which includes dynamc libraries too)
+@@ -259,7 +259,7 @@ endif
+ $(BINDIR)hello_world: $(RUST_SRC_HELLO) $(BINDIR)rustc $(LIBDIR)libstd.rlib
+ 	@mkdir -p $(dir $@)
+ 	@echo "[RUSTC] -o $@"
+-	$V$(DBG) $(BINDIR)rustc -L $(LIBDIR) $< -o $@
++	$V$(DBG) $(BINDIR)rustc --target $(RUSTC_TARGET) -C target-feature=-crt-static -C link-self-contained=no -L $(LIBDIR) $< -o $@
+ 	./$@
+ 
+ #.PHONY: TEST_BOOTSTRAP
 diff --git a/run_rustc/rustc_proxy.sh b/run_rustc/rustc_proxy.sh
 index 3f4f2b9..78f6f1d 100755
 --- a/run_rustc/rustc_proxy.sh