improve the build rules for installing /lib/ld-musl-$ARCH.so

these new rules should avoid spurious error messages when the
directory (usually /lib) and the dynamic linker symlink already exist,
and minimize the spam when they can't be created.

Makefile

@@ -124,10 +124,12 @@ $(DESTDIR)$(libdir)/%: lib/%
 $(DESTDIR)$(includedir)/%: include/%
 	install -D -m 644 $< $@
 
-$(DESTDIR)$(LDSO_PATHNAME): lib/libc.so
-	install -d -m 755 $(DESTDIR)$(syslibdir) || true
+$(DESTDIR)$(LDSO_PATHNAME): $(DESTDIR)$(syslibdir)
 	ln -sf $(libdir)/libc.so $@ || true
 
+$(DESTDIR)$(syslibdir):
+	install -d -m 755 $(DESTDIR)$(syslibdir)
+
 .PRECIOUS: $(CRT_LIBS:lib/%=crt/%)
 
 .PHONY: all clean install