runtime: add support for panic/recover in Plan 9 note handler

runtime: add support for panic/recover in Plan 9 note handler

This change also resolves some issues with note handling: we now make
sure that there is enough room at the bottom of every goroutine to
execute the note handler, and the `exitstatus' is no longer a global
entity, which resolves some race conditions.

R=rminnich, npe, rsc, ality
CC=golang-dev
https://golang.org/cl/6569068