cgo: In _cgo_main.c define all provided symbols as functions.
cgo: In _cgo_main.c define all provided symbols as functions.

This defines crosscall2 in a way that matches that real
definition, and also defines _cgo_allocate and _cgo_panic to
indicate that they are available to be called by code compiled
with gcc.

R=rsc
CC=golang-dev
http://codereview.appspot.com/4026041
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go