Introduce math_compat.h for older Android versions (#28567)
Summary:
When building with Android NDK platforms prior to android-21,
and when building for Android with libstdc++, there are some
gaps in the C and C++ standard libraries. We use both for our
internal 32-bit builds, so we need PyTorch to support this platform.
All of the gaps are filled with this math_compat.h header, which
needs to be included in any file that uses one of the functions
that are not properly defined on Android. The file is a bit
hack-tastic, but it is only used on a platform that is not receiving
updates, so there shouldn't be a risk of breakage in the future.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28567
Test Plan: Internal android build.
Differential Revision: D18099513
Pulled By: dreiss
fbshipit-source-id: 020aab19c6fa083206310b018925d92275d4a548
