Fix debugger printing of uint that mistakenly
invoked .int64() instead of .uint64()

Fixes #21392

Change-Id: I107a7e87e0efbb06303c1e627dee76c369f75d1e
Reviewed-on: https://go-review.googlesource.com/54750Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>