Summary:
SYS_gettid is different on Linux vs. OSX.  `__NR_gettid` is
only sometimes present and `SYS_gettid` is only sometimes present, but
we can pick one name and just follow that one.

Reviewed By: dcolascione

Differential Revision: D2800515

fb-gh-sync-id: 4245de4b9184ac4233ade9da297409c1031869a3