Correct _cgo_free when C ABI does not pass first arg on stack.

Correct _cgo_free when C ABI does not pass first arg on stack.

It turns out that _cgo_malloc is used, via cmalloc in
runtime/cgocall.c, which is called by code generated by out.go
for the ·_C_CString function.  I can't find a call to
_cgo_free, but given _cgo_malloc we might as well keep
_cgo_free.  This patch fixes it so that it should work on
amd64.

R=rsc
CC=golang-dev
https://golang.org/cl/1399041