runtime: use raise instead of pthread_self and pthread_kill

pthread_self and pthread_kill are not safe to call from a signal
handler. In particular, pthread_self fails in iOS when called from
a signal handler context.

Use raise instead; it is signal handler safe and simpler.

Change-Id: I0cbfe25151aed245f55d7b76719ce06dc78c6a75
Reviewed-on: https://go-review.googlesource.com/113877


Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>