cmd/cgo: handle another Clang DWARF quirk

cmd/cgo: handle another Clang DWARF quirk

Without the fix:

    $ CC=clang-3.5 ./test.bash
    misc/cgo/errors/test.bash: BUG: expected error output to contain "C.ushort" but saw:
    # command-line-arguments
    ./issue13129.go:13: cannot use int(0) (type int) as type C.unsignedshort in assignment

Fixes #13129.

Change-Id: I2c019d2d000f5bfa3e33c477e533aff97031a84f
Reviewed-on: https://go-review.googlesource.com/17207
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>