x/debug/dwarf: support new AttrGoEmbed field of struct members

The new custom attribute 0x2903 was added to DIEs of struct members to
distinguish struct embeds from regular fields.

CL: https://golang.org/cl/41873
Issue: https://github.com/golang/go/issues/20037

Change-Id: Ia676c6ef0d49e4e824e28d5cb53cad590c3b4f4a
Reviewed-on: https://go-review.googlesource.com/42512
Reviewed-by: John Dethridge <jcd@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>