Makefile: separate the target to create empty dummy libraries (WebAssembly#502)

* Makefile: separate the target to create empty dummy libraries

During develompment, it's sometimes convenient to have a separate target.

* ensure to create the destination directory

Author: yamt

Makefile

@@ -780,14 +780,16 @@ endif
 
 libc: include_dirs $(STATIC_LIBS)
 
-finish: startup_files libc
+dummy_libs:
 	#
 	# Create empty placeholder libraries.
 	#
+	mkdir -p "$(SYSROOT_LIB)" && \
 	for name in m rt pthread crypt util xnet resolv; do \
 	    $(AR) crs "$(SYSROOT_LIB)/lib$${name}.a"; \
 	done
 
+finish: startup_files libc dummy_libs
 	#
 	# The build succeeded! The generated sysroot is in $(SYSROOT).
 	#
@@ -984,4 +986,4 @@ clean:
 	$(RM) -r "$(OBJDIR)"
 	$(RM) -r "$(SYSROOT)"
 
-.PHONY: default startup_files libc libc_so finish install include_dirs clean check-symbols bindings
+.PHONY: default startup_files libc libc_so dummy_libs finish install include_dirs clean check-symbols bindings