Introduce folly::max_align_t and folly::max_align_v

Introduce folly::max_align_t and folly::max_align_v

Summary:
`folly::max_align_t` is a portable replacement for `std::max_align_t`.

`folly::max_align_v` is a `constexpr size_t` with the same value as `alignof(folly::max_align_t)`

32-bit iOS environments have `alignof(std::max_align_t) == 4`, which is not correct. This is more
correct in that I have not yet found a more-aligned basic type.

Reviewed By: yfeldblum

Differential Revision: D5442333

fbshipit-source-id: 0a93e48a730d65ef76e04f132b5976c93587b14c