Include reader / writer interactions of RWMutex in the mutex profile.
Writer contention is already included in the profile, since a plain Mutex
is used to control exclusion.

Fixes #18496

Change-Id: Ib0dc1ffa0fd5e6d964a6f7764d7f09556eb63f00
Reviewed-on: https://go-review.googlesource.com/87095
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Peter Weinberger <pjw@google.com>