runtime: rename p racectx field to raceprocctx

Both g and p had a racectx field, but they held different kinds of values.
The g field held ThreadState values while the p field held Processor values
(to use the names used in the C++ code in the compiler_rt support library).
Rename the p field to raceprocctx to reduce potential confusion.

Change-Id: Iefba0e259d240171e973054c452c3c15bf3f8f8f
Reviewed-on: https://go-review.googlesource.com/c/go/+/169960


Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>