runtime: use func value for parfor body

Yet another leftover from C: parfor took a func value for the
callback, casted it to an unsafe.Pointer for storage, and then casted
it back to a func value to call it.  This is unnecessary, so just
store the body as a func value.  Beyond general cleanup, this also
eliminates the last use of unsafe in parfor.

Change-Id: Ia904af7c6c443ba75e2699835aee8e9a39b26dd8
Reviewed-on: https://go-review.googlesource.com/3396
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

src/runtime/export_test.go

@@ -36,7 +36,7 @@ func LFStackPop(head *uint64) *LFNode {
 }
 
 type ParFor struct {
-	body   *byte
+	body   func(*ParFor, uint32)
 	done   uint32
 	Nthr   uint32
 	thrseq uint32