Remove libbacktrace definition of gettid.

Also simplifies the Darwin implementation of gettid, because
apparently libbacktrace had a better way of doing it.

Bug: 19517541

(cherry picked from commit 23f750b068ddf5b20fb5d153a798b22562368992)

Change-Id: I2f888e8ed7a2f5719973786cbcbb399a81867ee9
6 files changed