cmd/gc: fix handling of OGETG in race mode

cmd/gc: fix handling of OGETG in race mode

Now that getg is an intrinsic, more runtime functions
gets inlined (in particular, LockOSThread).
Runtime code gets race instrumented after inlining into
other packages. This can lead to false positives,
as race detector ignores all internal synchronization in runtime.
Inling of LockOSThread lead to false race reports on m contents.
See the issue for an example.

Fixes #10380

Change-Id: Ic9b760b53c28c2350bc54a5d4677fcd1c1f86e5f
Reviewed-on: https://go-review.googlesource.com/8690
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>