x86/mm: Implement native set_memory_rox()

Provide a native implementation of set_memory_rox(), avoiding the
double set_memory_ro();set_memory_x(); calls.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>