runtime: align first persistentalloc chunk as requested

Backport of upstream https://golang.org/cl/163859.

This fixes various failures on 32-bit SPARC.

Patch from Eric Boctazou.

Change-Id: I59312896574881f8d8ef58886ef9773f4865fb2d
Reviewed-on: https://go-review.googlesource.com/c/163860
Reviewed-by: Than McIntosh <thanm@google.com>
diff --git a/libgo/go/runtime/malloc.go b/libgo/go/runtime/malloc.go
index d2d8699..b6a7ee1 100644
--- a/libgo/go/runtime/malloc.go
+++ b/libgo/go/runtime/malloc.go
@@ -1269,7 +1269,7 @@
 				break
 			}
 		}
-		persistent.off = sys.PtrSize
+		persistent.off = round(sys.PtrSize, align)
 	}
 	p := persistent.base.add(persistent.off)
 	persistent.off += size