runtime: rename Lock to Mutex

Mutex is consistent with package sync, and when in the
unexported Go form it avoids having a conflcit between
the type (now mutex) and the function (lock).

LGTM=iant
R=golang-codereviews, iant
CC=dvyukov, golang-codereviews, r
https://golang.org/cl/133140043

src/pkg/runtime/malloc.c

@@ -350,7 +350,7 @@ runtime·MHeap_SysAlloc(MHeap *h, uintptr n)
 
 static struct
 {
-	Lock	lock;
+	Mutex	lock;
 	byte*	pos;
 	byte*	end;
 } persistent;