spec: fallthrough may not appear in last clause of a switch

Replacement for CL 7370053 which attempted to make fallthrough's
syntactically a part of switch statements. Because they may be
labeled, fixing that CL completely would require too much spec
surgery.

Fixes issue 4923.

R=r, iant, rsc, ken
CC=golang-dev
https://codereview.appspot.com/7416048
cmd/go: pass -intgosize to SWIG

swig >= 2.0.9 requires the size of int values to be passed via a command line flag. Should swig complain about the -intgosize not being supported, then alert the user to their outdated version of swig.

Fixes issue 4756.

R=golang-dev, minux.ma, iant
CC=golang-dev
https://codereview.appspot.com/7331048

Committer: Ian Lance Taylor <iant@golang.org>