internal/runtime/maps: use uintptr instead of uint32 for index in group

internal/runtime/maps: use uintptr instead of uint32 for index in group

This avoids some zero-extension ops on 64-bit machines.

Based on khr@'s CL 619479.

For #54766.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest-swissmap
Change-Id: Ie9a56da26382dc9e515c613abc8cf6fec3767671
Reviewed-on: https://go-review.googlesource.com/c/go/+/620216
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>