MFC r302517:

Fix a copy/paste bug introduced during X86_64 Linuxulator work.
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

Approved by: re (gjb)