runtime: Use atomic load for in entersyscall.
runtime: Use atomic load for in entersyscall.

Reportedly fixes PPC64 deadlock.

From a comment by Gabriel Russell.

Fixes golang/go#15051.

Change-Id: Ibdf13aa0e673ebcbd9323785a396204996e04070
Reviewed-on: https://go-review.googlesource.com/21450
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend