Added SAL annotatations to system calls.

Modify makesyscalls.sh to strip out SAL annotations.

No functional change.

This is based on work I started in CheriBSD and use to validate fat
pointers at the syscall boundary. Tal Garfinkel reviewed the changes,
added annotations to COMPAT* syscalls and is using them in a record and
playback framework. One can envision other uses such as a WITNESS-like
validator for copyin/out as speculated on in the review.

As this time we are only annotating sys/kern/syscalls.master as that is
sufficient for userspace work. If kernel use cases materialize, we can
annotate other syscalls.master as needed.

Submitted by: Tal Garfinkel <talg@cs.stanford.edu>
Sponsored by: DARPA, AFRL (in part)
Differential Revision: https://reviews.freebsd.org/D14285