MFC r349880:

Let linuxulator mprotect mask unsupported bits before calling kern_mprotect.

After r349240 kern_mprotect returns EINVAL for unsupported bits in the prot
argument. Linux rtld uses PROT_GROWSDOWN and PROT_GROWS_UP when marking the
stack executable. Mask these bits like kern_mprotect used to do. For other
unsupported bits EINVAL is returned like Linux does.

Reviewed by: trasz, brooks

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)