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

ssrl/go,apparentlymart/golang,ssrl/go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,ssrl/go,dsriram/golang,tv42/old-go,gnanderson/go,apparentlymart/golang,ssrl/go,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,ssrl/go,dsriram/golang,ssrl/go,gnanderson/go,dsriram/golang,tv42/old-go,ssrl/go,gnanderson/go,gnanderson/go,apparentlymart/golang,tv42/old-go,dsriram/golang,gnanderson/go,apparentlymart/golang,dsriram/golang