runtime: rename gothrow to throw

Rename "gothrow" to "throw" now that the C version of "throw"
is no longer needed.

This change is purely mechanical except in panic.go where the
old version of "throw" has been deleted.

sed -i "" 's/[[:<:]]gothrow[[:>:]]/throw/g' runtime/*.go

Change-Id: Icf0752299c35958b92870a97111c67bcd9159dc3
Reviewed-on: https://go-review.googlesource.com/2150
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>