[libcxx] fix incorrect attribute property

Summary:
`__has_attribute(fallthough)` -> `__has_attribute(fallthrough)`
This is a follow-up of https://reviews.llvm.org/D72287

Reviewers: EricWF, mclow.lists, Jim

Reviewed By: Jim

Subscribers: christof, ldionne, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D72314