[pytorch][cmake] improve build mobile with host toolchain (#34187)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34187
Noticed that a recent PR broke Android/iOS CI but didn't break mobile
build with host toolchain. Turns out one mobile related flag was not
set on PYTORCH_BUILD_MOBILE code path:
```
"set(INTERN_DISABLE_MOBILE_INTERP ON)"
```
First, move the INTERN_DISABLE_MOBILE_INTERP macro below, to stay with
other "mobile + pytorch" options - it's not relevant to "mobile + caffe2"
so doesn't need to be set as common "mobile" option;
Second, rename PYTORCH_BUILD_MOBILE env-variable to
BUILD_PYTORCH_MOBILE_WITH_HOST_TOOLCHAIN - it's a bit verbose but
becomes more clear what it does - there is another env-variable
"BUILD_PYTORCH_MOBILE" used in scripts/build_android.sh, build_ios.sh,
which toggles between "mobile + pytorch" v.s. "mobile + caffe2";
Third, combine BUILD_PYTORCH_MOBILE_WITH_HOST_TOOLCHAIN with ANDROID/IOS
to avoid missing common mobile options again in future.
Test Plan: Imported from OSS
Differential Revision: D20251864
Pulled By: ljk53
fbshipit-source-id: dc90cc87ffd4d0bf8a78ae960c4ce33a8bb9e912
