runtime: skip stack growth test on 32bit platforms
runtime: skip stack growth test on 32bit platforms

Update issue 8083

See discussion in https://groups.google.com/forum/#!topic/golang-dev/dh6Ra_xJomc

LGTM=khr
R=golang-codereviews, gobot, khr
CC=golang-codereviews
https://codereview.appspot.com/99440048

pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme