builtin: spec correction for panic()

Upon unrecovered panic and program termination, process exit code is hard-coded
to 2, not set to the parameter passed to panic().

Change-Id: If64b75493227b4fd69c0bbb529f84e6df2d1b93f
Reviewed-on: https://go-review.googlesource.com/c/go/+/167709


Reviewed-by: Robert Griesemer <gri@golang.org>