libc++/win: Make once_flag have the same size as a pointer.

`unsigned long` is 32-bit on 32-bit systems and 64-bit on 64-bit systems
on LP64 systems -- which most Unix systems are, but Windows isn't.
Windows is LLP64, which means unsigned long is 32-bit even on 64-bit
systems.

pplwin.h contains

    static_assert(alignof(void *) == alignof(::std::once_flag), ...)

which fails due to this problem.

Instead of unsigned long, use uintptr_t, which consistently is 32-bit
on 32-bit systems and 64-bit on 64-bit systems.

No functional change except on 64-bit Windows.

Differential Revision: https://reviews.llvm.org/D59607

llvm-svn: 356624