mm: Convert __HAVE_ARCH_P..P_GET to the new style

Since __HAVE_ARCH_* style guards have been depricated in favour of
defining the function name onto itself, convert pxxp_get().

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/Y2EUEBlQXNgaJgoI@hirez.programming.kicks-ass.net