ninja/mac: Fix multiarch link regression caused by 1871.

The bug was caused by i386 and x86_64 binary using the same rsp file.
Add arch to rsp filename in multi-arch builds.

(ld64 prints a warning about .o files not having the right arch when linking
the 64bit version using the rsp file written for the 32bit link and the other
way round and silently skips the .o file, but it exits with 0. To test for this,
get symbols from the final fat binary in each arch and check that they are all
there -- this fails fairly reliably without the patch, even though it depends
on process invocation order.)

BUG=chromium:352816
TBR=scottmg@chromium.org (lg'd at https://codereview.chromium.org/200943003/)

Review URL: https://codereview.chromium.org/196283023

git-svn-id: http://gyp.googlecode.com/svn/trunk@1874 78cadc50-ecff-11dd-a971-7dbc132099af