runtime: convert p.numTimers and p.deletedTimers to internal atomic types

runtime: convert p.numTimers and p.deletedTimers to internal atomic types

Note that this changes the non-atomic operations in p.destroy() to atomic operations.

For #53821

Change-Id: I7bba77c9a2287ba697c87cce2c79293e4d1b3334
Reviewed-on: https://go-review.googlesource.com/c/go/+/425774
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>