[libcxx] [test] Generalize the conditions for testing bitcasts between long...

[libcxx] [test] Generalize the conditions for testing bitcasts between long double, double and int128

MSVC targets also have a 64 bit long double, as do MinGW targets on ARM.
This hasn't been noticed in CI because the MSVC configurations there run
with _LIBCPP_HAS_NO_INT128 defined.

This avoids assuming that either __int128_t or double is equal in size to
long double. i386 MinGW targets have sizeof(long double) == 10, which
doesn't match any of the tested types.

Differential Revision: https://reviews.llvm.org/D111671