cmd/cgo: mishandles 40 bit bitfield

Building this test case with `go build foo.go`

```
package p

/*
typedef struct {
	unsigned long x : 40;
} s;
*/
import "C"

var v C.s
```

produces

```
# command-line-arguments
/tmp/go-build300185653/b001/_cgo_gotypes.go:23:4: undefined: uint40
```

Oddly, something seems to be special about the number `40`.