cmd/go: clean cgo compiler errors

cmd/go: clean cgo compiler errors

Cut out temporary cgo file in error message.
Show C.foo instead of _Ctype_foo.

Before:
x.go:20[/var/folders/00/05_b8000h01000cxqpysvccm000n9d/T/go-build242036121/command-line-arguments/_obj/x.cgo1.go:19]: cannot use tv.Usec (type int32) as type _Ctype___darwin_suseconds_t in assignment

After:
x.go:20: cannot use tv.Usec (type int32) as type C.__darwin_suseconds_t in assignment

Fixes #4255.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/7231075