Use `gettimeofday` on iOS (#30361)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30361
### Summary
By default, the compiler will choose `clock_gettime` for the iOS build. However, that API is not available until iOS 10. Since the Facebook app still supports iOS 9.0, we have to use `gettimeofday` instead.
```shell
xplat/caffe2/torch/csrc/autograd/profiler.h:86:3: error: 'clock_gettime' is only available on iOS 10.0 or newer [-Werror,-Wunguarded-availability]
xplat/caffe2/torch/csrc/autograd/profiler.h:86:17: error: '_CLOCK_MONOTONIC' is only available on iOS 10.0 or newer [-Werror,-Wunguarded-availability]
```
P.S. the open-sourced version is iOS 12.0 and above, so we don't have this problem.
### Test Plan
- buck build works
- Don't break CIs
Test Plan: Imported from OSS
Differential Revision: D18730262
Pulled By: xta0
fbshipit-source-id: fe6d954b8d3c23cbc9d1e25a2e72e0b0c1d4eaa9
