[libcxxabi] Fix layout of __cxa_exception for win64

Win64 isn't LP64, it's LLP64, but there's no __LLP64__ predefined -
just check _WIN64 in addition to __LP64__.

This fixes compilation after static asserts about the struct layout
were added in f2a436058fcbc11291e73badb44e243f61046183.

Differential Revision: https://reviews.llvm.org/D73838

GitOrigin-RevId: 09dc884eb2e4a433eb8c5ed20a17108091279295
1 file changed