[libc++] Do not define std::nothrow with VCRuntime ABI (#217694)

Avoid defining `std::nothrow` in libc++ when `_LIBCPP_ABI_VCRUNTIME` is
enabled.

VCRuntime already owns and provides `std::nothrow`, just as it owns the
global new/delete ABI.

Assisted-by: codex (for the test)
GitOrigin-RevId: d291bcd466edac6b0e21b6f6437462d763e337de