runtime/cgo: Don't define crosscall2 in dummy _cgo_main.c.
runtime/cgo: Don't define crosscall2 in dummy _cgo_main.c.

In this specific package crosscall2 is already defined in a .S
file anyhow.  This avoids a warning about mismatched
alignment.

R=rsc
CC=golang-dev
http://codereview.appspot.com/4000043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang