Current ubsan complains on every use of __PTR_ALIGN (when ptrdiff_t is
as large as a pointer), due to making calculations relative to a NULL
pointer.  This patch avoids the problem by extracting out and
simplifying __BPTR_ALIGN for the usual case.  I've continued to use
ptrdiff_t here, where it might be better to throw away __BPTR_ALIGN
entirely and just assume uintptr_t exists.

	* obstack.h (__PTR_ALIGN): Expand and simplify __BPTR_ALIGN
	rather than calculating relative to a NULL pointer.