Linux: Use kill_lwp/tkill instead of kill when killing a process

Since we use tkill everywhere, using kill to try to kill each lwp
individually looks suspiciously odd.  We should really be using tgkill
everywhere, but at least while we don't get there this makes us
consistent.

gdb/gdbserver/
2014-07-16  Pedro Alves  <palves@redhat.com>

	* linux-low.c (linux_kill_one_lwp): Use kill_lwp, not kill.

gdb/
2014-07-16  Pedro Alves  <palves@redhat.com>

	* linux-nat.c (kill_callback): Use kill_lwp, not kill.