[xray] Detect MPROTECT and error out when it's enabled (on NetBSD)

Add a CheckMPROTECT() routine to detect when pax MPROTECT is enabled
on NetBSD, and error xray out when it is.  The solution is adapted
from existing CheckASLR().

Differential Revision: https://reviews.llvm.org/D56049

llvm-svn: 350030