build: mac OBJ_DIR should point to obj.target

I think there might be an issue with the value of OBJ_DIR when
using a "mac" os. The value is currently specified in common.gypi
which is included by node.gyp:
'OBJ_DIR': '<(PRODUCT_DIR)/obj',

In the generated Makefile (out/Makefile) the object output directory
is:
obj := $(builddir)/obj

And in the included node.target.mk we have the OBJS declared:
OBJS := \
         $(obj).target/$(TARGET)/src/async-wrap.o \
         $(obj).target/$(TARGET)/src/cares_wrap.o \

If OBJ_DIR is used in node.gyp to point to generated object files
on mac they will not be found.

PR-URL: #11857
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: danbev

common.gypi

@@ -43,7 +43,7 @@
         'v8_postmortem_support%': 'true',
       }],
       ['OS== "mac"', {
-        'OBJ_DIR': '<(PRODUCT_DIR)/obj',
+        'OBJ_DIR': '<(PRODUCT_DIR)/obj.target',
         'V8_BASE': '<(PRODUCT_DIR)/libv8_base.a',
       }, {
         'conditions': [