FreeBSD support NX bit on X86_64 processors out of the box, for i386 emulation
use READ_IMPLIES_EXEC flag, introduced in r302515.
While here move common part of mmap() and mprotect() code to the files in compat/linux
to reduce code dupcliation between Linuxulator's.
Reported by: Johannes Jost Meixner, Shawn Webb
MFC after: 1 week
XMFC with: r302515, r302516
