Add ARM64 support. (#1191)
Add ARM64 support. (#1191)

Summary:
Allow builds for 64-bit ARM (MSVC) to use the same implementation of `__builtin_clzll` as ARM and x86 architectures.
Pull Request resolved: https://github.com/facebook/folly/pull/1191

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Reviewed By: yfeldblum

Differential Revision: D16350573

Pulled By: Orvid

fbshipit-source-id: f25087b3a53de09767f505ee8c7d971a6d957189

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly