runtime: convert g.goid to uint64

runtime: convert g.goid to uint64

schedt.goidgen and p.goidcache are already uint64, this makes all cases
consistent.

The only oddball here is schedtrace which prints -1 as an equivalent for
N/A or nil. A future CL will make this more explicit.

Change-Id: I489626f3232799f6ca333d0d103b71d9d3aa7494
Reviewed-on: https://go-review.googlesource.com/c/go/+/419440
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>