internal/runtime/maps: prune tombstones in maps before growing

internal/runtime/maps: prune tombstones in maps before growing

Before growing, if there are lots of tombstones try to remove them.
If we can remove enough, we can continue at the given size for a
while longer.

Fixes #70886

Change-Id: I71e0d873ae118bb35798314ec25e78eaa5340d73
Reviewed-on: https://go-review.googlesource.com/c/go/+/640955
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>