Revert "runtime: redo heap bitmap"

This reverts commit b589208c8c.

Reason for revert: Bug somewhere in this code, causing wasm and maybe linux/386 to fail.

Change-Id: I5e1e501d839584e0219271bb937e94348f83c11f
Reviewed-on: https://go-review.googlesource.com/c/go/+/422395
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Keith Randall <khr@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/runtime/mcache.go

@@ -251,7 +251,7 @@ func (c *mcache) allocLarge(size uintptr, noscan bool) *mspan {
 	// visible to the background sweeper.
 	mheap_.central[spc].mcentral.fullSwept(mheap_.sweepgen).push(s)
 	s.limit = s.base() + size
-	s.initHeapBits()
+	heapBitsForAddr(s.base()).initSpan(s)
 	return s
 }