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