fix makefile suppression of intermediate file removal

at one point, GNU make was removing crt/*.o after producing the copies
in lib/ due to an arcane misfeature for handling "intermediate" files.
the circumstances that caused this are no longer present in our
makefile, but the previous workaround using .PRECIOUS was wrong and
could result in corrupt/partial files being left behind during an
interrupted build. using .SECONDARY is the correct, documented fix
that will prevent deletion of "intermediate" files from ever
resurfacing.

Makefile

@@ -203,6 +203,6 @@ musl-git-%.tar.gz: .git
 musl-%.tar.gz: .git
 	 git archive --format=tar.gz --prefix=$(patsubst %.tar.gz,%,$@)/ -o $@ v$(patsubst musl-%.tar.gz,%,$@)
 
-.PRECIOUS: $(CRT_LIBS:lib/%=crt/%)
+.SECONDARY:
 
 .PHONY: all clean install install-libs install-headers install-tools