cgo: omit duplicate symbols in writeDefs

cgo: omit duplicate symbols in writeDefs

When the C API being used includes multiple names for the same
underlying symbol (e.g. multiple #define's for the same variable), then
cgo will generate the same placeholder variables for each name.  This
then prevents the code from compiling due to multiple declarations of
the same variable - so change cgo to only create one instance of the
variable for the underlying symbol.

R=rsc
CC=golang-dev
https://golang.org/cl/4826055