syscall-emul: Add the tgkill system call

syscall-emul: Add the tgkill system call

This changeset adds support to kill a thread group by calling
the tgkill system call. The functionality is needed in some
pthread applications.

Change-Id: I0413a3331be69b74dfab30de95384113ec4efb63
Reviewed-on: https://gem5-review.googlesource.com/2268
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Tony Gutierrez <anthony.gutierrez@amd.com>
Reviewed-by: Michael LeBeane <Michael.Lebeane@amd.com>