ninja/mac: Try to fix 'IOError: [Errno 2] No such file or directory' error.

ninja/mac: Try to fix 'IOError: [Errno 2] No such file or directory' error.

Happening on this Blink bot:
http://chromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Mac%20Builder%20%28dbg%29

This probably broke when we changed the generator to write ninja files after
their contents are known -- because of that the folder doesn't exist yet
when the per-arch files are written.

R=scottmg@chromium.org

Committed: https://code.google.com/p/gyp/source/detail?r=1726

[scottmg, 2013-09-12 19:56:15]

lgtm

[Nico, 2013-09-12 20:02:23]

Committed patchset #1 manually as r1726 (presubmit successful).