Fix lwp_suspend/unsuspend imbalance in linux_wait_1

Fix lwp_suspend/unsuspend imbalance in linux_wait_1

This patch fixes imbalanced lwp_suspend/unsuspend calls caused by the
premature choosing of another event for fairness.

select_event_lwp would switch the event before a call to
unsuspend_all_lwps, thus it would be called with the wrong event.

This caused an assertion failure: unsuspend LWP xx, suspended=-1 when
testing  gdb.threads/non-stop-fair-events.exp with ARM range stepping in
GDBServer.

This patch moves the switch of event after the unsuspend/unstop calls.

No regressions, tested on ubuntu 14.04 ARMv7 and x86.
With gdbserver-native.

gdb/gdbserver/ChangeLog:

	* linux-low.c (linux_wait_1): Move event switch after unsuspend_lwps.