runtime: measure speed of procyield and osyield

These are delay primitives for lock2. If a mutex isn't immediately
available, we can use procyield to tell the processor to wait for a
moment, or osyield to allow the OS to run a different process or thread
if one is waiting. We expect a processor-level yield to be faster than
an os-level yield, and for both of them to be fast relative to entering
a full sleep (via futexsleep or semasleep).

Each architecture has its own way of hinting to the processor that it's
in a spin-wait loop, so procyield presents an architecture-independent
interface for use in lock_futex.go and lock_sema.go.

Measure the (single-threaded) speed of these to confirm.

For #68578

Change-Id: I90cd46ea553f2990395aceb048206285558c877e
Reviewed-on: https://go-review.googlesource.com/c/go/+/601396
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

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