When we have already assigned the semaphore ticket to a specific
waiter, we want to get the waiter running as fast as possible since
no other G waiting on the semaphore can acquire it optimistically.

The net effect is that, when a sync.Mutex is contended, the code in
the critical section guarded by the Mutex gets a priority boost.

Fixes #33747

The original work was done in CL 200577 by Carlo Alberto Ferraris. The
change was reverted in CL 205817 because it broke the linux-arm64-packet
and solaris-amd64-oraclerel builders.

Change-Id: I76d79b1d63fd206ed1c57fe6900cb7ae9e4d46cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/206180
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>