sysent: Add a sv_psstringssz field to struct sysentvec

The size of the ps_strings structure varies between ABIs, so this is
useful for computing the address of the ps_strings structure relative to
the top of the stack when stack address randomization is enabled.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 3fc21fdd5f8a4eeb71f43cc6841f59ef1debaf9a)