Remove hack in __gthread_active_p() to always return 0 on x86_64

This was causing __cxa_guard_acquire to fail and abort if
two threads attempted to initialise the same static local
at same time.

It also presumably had other adverse effects in other
parts of the gcc standard libraries.

TEST= local testing + trybots
BUG= https://code.google.com/p/nativeclient/issues/detail?id=3948
R=mcgrathr@chromium.org

Review URL: https://codereview.chromium.org/841313002
1 file changed