cmd/cgo: use first error position instead of last one

cmd/cgo: use first error position instead of last one

Just like https://golang.org/cl/34783

Given cgo.go:
     1	package main
     2
     3	/*
     4	long double x = 0;
     5	*/
     6	import "C"
     7
     8	func main() {
     9		_ = C.x
    10		_ = C.x
    11	}

Before:
    ./cgo.go:10:6: unexpected: 16-byte float type - long double

After:
    ./cgo.go:9:6: unexpected: 16-byte float type - long double

The above test case is not portable. So it is tested on only amd64.

Change-Id: If0b84cf73d381a22e2ada71c8e9a6e6ec77ffd2e
Reviewed-on: https://go-review.googlesource.com/54950
Reviewed-by: Ian Lance Taylor <iant@golang.org>