GCM allows using tag sizes smaller than the block size. This adds a
NewGCMWithNonceAndTagSize function which allows specifying the tag
size.

Fixes #19594

Change-Id: Ib2008c6f13ad6d916638b1523c0ded8a80eaf42d
Reviewed-on: https://go-review.googlesource.com/48510Reviewed-by: Filippo Valsorda <hi@filippo.io>
Run-TryBot: Filippo Valsorda <hi@filippo.io>
TryBot-Result: Gobot Gobot <gobot@golang.org>