linux: populate sv_syscallnames in each sysentvec

This allows the syscallname() function to give a usable result for Linux
ABIs.

Reported by: jrtc27
Reviewed by: jrtc27, markj, jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37199

(cherry picked from commit 1da65dcb1c57fae7be75ddf1bd300ddf19ced850)
(cherry picked from commit f396f9b6c96f6d68d4e3606ddb090287230fc565)