Introduces the new relocation variant RV_390_DBL which indicates
that the relocation value should be shifted right by 1 (to make
it 2-byte aligned).

Change-Id: I03fa96b4759ee19330c5298c3720746622fb1a03
Reviewed-on: https://go-review.googlesource.com/20878
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>