Fix folly::max_align_v for Clang on ARMv7
Summary:
[Folly] Fix `folly::max_align_v` for Clang on ARMv7.
There is some problem with computing the correct result of `alignof` for a compound `union` POD type, because why wouldn't there be. The result *should* just be the max of the `alignof` of the type of each field if each field is default aligned - but not on some platforms! So we must compute the max directly.
Reviewed By: mzlee, Orvid
Differential Revision:
D6573548
fbshipit-source-id:
512a255fda64795104d71fde14372befa3bf41e4