linux: implement PTRACE_GET_SYSCALL_INFO

This is one of the pieces required to make modern (ie Focal)
strace(1) work.

Reviewed By: jhb (earlier version)
Sponsored by: EPSRC
Differential Revision: https://reviews.freebsd.org/D28212

(cherry picked from commit bdf0f24bb16d556a5b1e01cdfc087d08e91ac572)