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

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