cmd/compile,reflect: allow longer type names

Encode the length of type names and tags in a varint encoding
instead of a fixed 2-byte encoding. This allows lengths longer
than 65535 (which can happen for large unnamed structs).

Removed the alignment check for #14962, it isn't relevant any more
since we're no longer reading pointers directly out of this data
(it is encoded as an offset which is copied out bytewise).

Fixes #44155
Update #14962

Change-Id: I6084f6027e5955dc16777c87b0dd5ea2baa49629
Reviewed-on: https://go-review.googlesource.com/c/go/+/318249


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>