Remove strange v8_target_arch=="mac" conditions from gyp files

mac is not an architecture, and it doesn't make sense to compile in both
the ia32 and x64 archs at the same time

BUG=none
R=jkummerow@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/255183002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21176 ce2b1a6d-e550-0410-aec6-3dcde31c8c00