newlib: fix multilib libg.a parallel builds

I split libg.a out into a sep target from libc.a for the main dir in
commit f2b053f49e ("newlib: separate out
libg from libc"), but missed the multilib dirs.  That leads to an
uncommon parallel build failure:
- libc.a rule runs & finishes
- $(BUILD_MULTISUBDIR)/libc.a rule runs
  -> failure due to libg.a not yet existing
- libg.a rule runs & finishes

Split the multilib libg rule out from libc too so it can depend on the
main libg directly and avoid this race.

newlib/Makefile.am

@@ -161,10 +161,12 @@ $(BUILD_MULTISUBDIR)/crt0.o: $(CRT0_DIR)$(CRT0) $(BUILD_MULTISUBDIR)
 	ln $< $@ >/dev/null 2>/dev/null || cp $< $@
 
 $(BUILD_MULTISUBDIR)/libc.a: libc.a $(BUILD_MULTISUBDIR)
-	rm -f $@ $(BUILD_MULTISUBDIR)/libg.a
+	rm -f $@
+	ln $< $@ >/dev/null 2>/dev/null || cp $< $@
+
+$(BUILD_MULTISUBDIR)/libg.a: libg.a ${BUILD_MULTISUBDIR}
+	rm -f $@
 	ln $< $@ >/dev/null 2>/dev/null || cp $< $@
-	ln libg.a $(BUILD_MULTISUBDIR)/libg.a >/dev/null 2>/dev/null || \
-	cp libg.a $(BUILD_MULTISUBDIR)/libg.a
 
 $(BUILD_MULTISUBDIR)/libm.a: libm.a $(BUILD_MULTISUBDIR)
 	rm -f $@

newlib/Makefile.in

@@ -8672,10 +8672,12 @@ libm.a: $(libm_a_OBJECTS) $(libm_a_DEPENDENCIES)
 @HAVE_MULTISUBDIR_TRUE@	ln $< $@ >/dev/null 2>/dev/null || cp $< $@
 
 @HAVE_MULTISUBDIR_TRUE@$(BUILD_MULTISUBDIR)/libc.a: libc.a $(BUILD_MULTISUBDIR)
-@HAVE_MULTISUBDIR_TRUE@	rm -f $@ $(BUILD_MULTISUBDIR)/libg.a
+@HAVE_MULTISUBDIR_TRUE@	rm -f $@
+@HAVE_MULTISUBDIR_TRUE@	ln $< $@ >/dev/null 2>/dev/null || cp $< $@
+
+@HAVE_MULTISUBDIR_TRUE@$(BUILD_MULTISUBDIR)/libg.a: libg.a ${BUILD_MULTISUBDIR}
+@HAVE_MULTISUBDIR_TRUE@	rm -f $@
 @HAVE_MULTISUBDIR_TRUE@	ln $< $@ >/dev/null 2>/dev/null || cp $< $@
-@HAVE_MULTISUBDIR_TRUE@	ln libg.a $(BUILD_MULTISUBDIR)/libg.a >/dev/null 2>/dev/null || \
-@HAVE_MULTISUBDIR_TRUE@	cp libg.a $(BUILD_MULTISUBDIR)/libg.a
 
 @HAVE_MULTISUBDIR_TRUE@$(BUILD_MULTISUBDIR)/libm.a: libm.a $(BUILD_MULTISUBDIR)
 @HAVE_MULTISUBDIR_TRUE@	rm -f $@