offer REALGCC variable to configure musl-gcc wrapper at runtime

this is useful when the underlying gcc is already a wrapper, which is
the case at least on some uclibc-based system images. it's also useful
for running an older/newer/nondefault version of gcc.

Makefile

@@ -112,7 +112,7 @@ lib/musl-gcc.specs: tools/musl-gcc.specs.sh config.mak
 	sh $< "$(includedir)" "$(libdir)" "$(LDSO_PATHNAME)" > $@
 
 tools/musl-gcc: config.mak
-	printf '#!/bin/sh\nexec gcc "$$@" -specs "%s/musl-gcc.specs"\n' "$(libdir)" > $@
+	printf '#!/bin/sh\nexec "$${REALGCC:-gcc}" "$$@" -specs "%s/musl-gcc.specs"\n' "$(libdir)" > $@
 	chmod +x $@
 
 $(DESTDIR)$(bindir)/%: tools/%