runtime: don't call lockOSThread for every cgo call

For a trivial benchmark with a do-nothing cgo call:

name    old time/op  new time/op  delta
Call-4  64.5ns ± 7%  63.0ns ± 6%  -2.25%  (p=0.027 n=20+16)

Because Windows uses the cgocall mechanism to make system calls,
and passes arguments in a struct held in the m,
we need to do the lockOSThread/unlockOSThread in that code.

Because deferreturn was getting a nosplit stack overflow error,
change it to avoid calling typedmemmove.

Updates #21827.

Change-Id: I9b1d61434c44faeb29805b46b409c812c9acadc2
Reviewed-on: https://go-review.googlesource.com/64070
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/runtime/panic.go

@@ -273,7 +273,17 @@ func freedefer(d *_defer) {
 			unlock(&sched.deferlock)
 		})
 	}
-	*d = _defer{}
+
+	// These lines used to be simply `*d = _defer{}` but that
+	// started causing a nosplit stack overflow via typedmemmove.
+	d.siz = 0
+	d.started = false
+	d.sp = 0
+	d.pc = 0
+	d.fn = nil
+	d._panic = nil
+	d.link = nil
+
 	pp.deferpool[sc] = append(pp.deferpool[sc], d)
 }