Fix ios_crash:backtrace=FBCameraFramework:caffe2::getClockTimeMilliseconds() (perf_observer.cc (#24813)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24813
clock_gettime does not work on some apple platforms e.g. OSX < 10.12
Use a custom implementation, similar to https://opensource.apple.com/source/Libc/Libc-1158.1.2/gen/clock_gettime.c.auto.html
T52655182
Test Plan: sandcastle tests
Differential Revision: D16883407
fbshipit-source-id: a42828bb91bb0c43297e9bdce4b18f7c9ea4274d
