[release-branch.go1.11] runtime: never call into race detector with retaken P
    
    cgocall could previously invoke the race detector on an M whose P had
    been retaken. The race detector would attempt to use the P-local state
    from this stale P, racing with the thread that was actually wired to
    that P. The result was memory corruption of ThreadSanitizer's internal
    data structures that presented as hard-to-understand assertion failures
    and segfaults.
    
    Reorder cgocall so that it always acquires a P before invoking the race
    detector, and add a test that stresses the interaction between cgo and
    the race detector to protect against future bugs of this kind.
    
    Fixes #28690.
    
    Change-Id: Ide93f96a23490314d6647547140e0a412a97f0d4
    Reviewed-on: https://go-review.googlesource.com/c/148717
    Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
    Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
    (cherry picked from commit e496e612b7f45a09209f8f4e1c7c1d0db378dc18)
    Reviewed-on: https://go-review.googlesource.com/c/148902
    Run-TryBot: Ian Lance Taylor <iant@golang.org>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Ian Lance Taylor <iant@golang.org>