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/stack.c

@@ -32,7 +32,7 @@ enum
 //     order = log_2(size/FixedStack)
 // There is a free list for each order.
 static MSpan stackpool[NumStackOrders];
-static Lock stackpoolmu;
+static Mutex stackpoolmu;
 // TODO: one lock per order?
 
 void