No test as the only system I know that uses 64-bit DWARF is AIX.

Change-Id: I24e225253075be188845656b6778993c2d24ebf5
Reviewed-on: https://go-review.googlesource.com/84379
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Austin Clements <austin@google.com>