[libcxx] Mark a test as unsupported for C++03

The nullptr_t_integral_cast.pass.cpp test is currently xfailed for
C++03, but actually, it only fails with the first version of libc++
ABI.

This patch changes XFAIL to UNSUPPORTED to avoid unexpected passes
with ABI v2 or later.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D93941