patch: runtime: TailscaleCurrentP

[bradfitz]

For tailscale/tailscale#14412 for sharded counters that have high write load, we want a ~"current CPU" hint, even if it's outdated by the time we use it.

+func TailscaleCurrentP() int {
+       return int(getg().m.p.ptr().id)
+}

Commit 161c3b7

/cc @raggi

[bradfitz]

ts1.24: 320c0ca