[libc++] __int128 support in type_traits

On Windows, what to do about sizeof(long double) for MSVC being 8 and > sizeof(long double) for g++ being 16.
s/16/12

12 is for g++ on 32 bit systems, but it’s 16 for 64 bit systems like mine. By default it’s 8 for MS’s cl.exe in both cases.