[libc++abi] Use std::nullptr_t instead of declaring it manually

Sometimes libc++'s stddef.h wrapper gets included, which defines
::nullptr_t. This test is compiled with -Wshadow -Werror, so shadowing
::nullptr_t with a nullptr_t in main is an error. Include cstddef,
which is guaranteed to define std::nullptr_t in C++11 and forward.

Reviewed By: ldionne, #libc_abi

Differential Revision: https://reviews.llvm.org/D137127