cmd/cgo: fix Xcode 5 incompatibility for #defined expressions

Ensure that clang always exits with a non-zero status by
giving it something that it always warns about (the statement "1;").

Fixes issue 6128.

R=golang-dev, iant, minux.ma
CC=golang-dev
https://codereview.appspot.com/14702043
undo CL 14231047 / 2f4c2dde2756

undone because the change slows down profile collection
significantly and unpredictable at times (see comments
at https://codereview.appspot.com/14231047 for details)

??? original CL description
runtime: collect profiles even while on g0 stack

Fixes issue 6417

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/14231047
???

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/14535046

Committer: Russ Cox <rsc@golang.org>