cgo: improve error reporting slightly.

cgo: improve error reporting slightly.
If there were warnings or errors in the user code,
cgo would print the first error from gcc and then stop,
which is not helpful.
This CL makes cgo ignore errors from user code
in the first pass - they will be shown later.
It also prints errors from user preamble code
with the correct line numbers.
(Also fixed misleading usage message).

R=iant, rsc
CC=golang-dev
https://golang.org/cl/4082047