runtime: align first persistentalloc chunk as requested

Change-Id: Ib391e019b1a7513d234fb1c8ff802efe8fa7c950
Reviewed-on: https://go-review.googlesource.com/c/go/+/163859
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/malloc.go

@@ -1248,7 +1248,7 @@ func persistentalloc1(size, align uintptr, sysStat *uint64) *notInHeap {
 				break
 			}
 		}
-		persistent.off = sys.PtrSize
+		persistent.off = round(sys.PtrSize, align)
 	}
 	p := persistent.base.add(persistent.off)
 	persistent.off += size