Support the N_PEXT bit of the Mach-O object format.
This change allows us to set the N_PEXT bit to Mach-O object files when a global symbol has a 'private_extern' extension.

BUG=http://www.tortall.net/projects/yasm/ticket/236
TEST=none
Review URL: http://codereview.chromium.org/6347060

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/yasm/patched-yasm@73761 4ff67af0-8c30-449e-8e8b-ad334ec8d88c