[flang] Restrict __float128 support for some build configurations.

This change is intended to resolve build issues reported in D134503.
A compiler supporting __float128 must define either __FLOAT128__ or
__SIZEOF_FLOAT128__ (or both). Additional check for _LIBCPP_VERSION
was added to disable __float128 for builds with libc++, because
__float128 support is incomplete there.

Differential Revision: https://reviews.llvm.org/D136121