runtime: make mcache.tiny a uintptr

mcache.tiny is in non-GC'd memory, but points to heap memory. As a
result, there may or may not be write barriers when writing to
mcache.tiny. Make it clearer that funny things are going on by making
mcache.tiny a uintptr instead of an unsafe.Pointer.

Change-Id: I732a5b7ea17162f196a9155154bbaff8d4d00eac
Reviewed-on: https://go-review.googlesource.com/16963
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/mcache.go

@@ -27,7 +27,7 @@ type mcache struct {
 	// tiny is a heap pointer. Since mcache is in non-GC'd memory,
 	// we handle it by clearing it in releaseAll during mark
 	// termination.
-	tiny             unsafe.Pointer
+	tiny             uintptr
 	tinyoffset       uintptr
 	local_tinyallocs uintptr // number of tiny allocs not counted in other stats
 
@@ -139,6 +139,6 @@ func (c *mcache) releaseAll() {
 		}
 	}
 	// Clear tinyalloc pool.
-	c.tiny = nil
+	c.tiny = 0
 	c.tinyoffset = 0
 }