internal/runtime/maps: proper capacity hint handling

internal/runtime/maps: proper capacity hint handling

When given a hint size, set the initial capacity large enough to avoid
requiring growth in the average case.

When not given a hint (or given 0), don't allocate anything at all.

For #54766.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest-swissmap
Change-Id: I8844fc652b8d2d4e5136cd56f7e78999a07fe381
Reviewed-on: https://go-review.googlesource.com/c/go/+/616457
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>