Replace NULL with nullptr

Also fixed some lines that were too long, and a few other minor
details.

Change-Id: I6efba5fb6e03eb5d0a300fddb2a75bf8e2f175cb
diff --git a/runtime/arch/arm64/context_arm64.h b/runtime/arch/arm64/context_arm64.h
index f486779..0383ad6 100644
--- a/runtime/arch/arm64/context_arm64.h
+++ b/runtime/arch/arm64/context_arm64.h
@@ -80,7 +80,7 @@
   NO_RETURN void DoLongJump() OVERRIDE;
 
  private:
-  // Pointers to register locations, initialized to NULL or the specific registers below.
+  // Pointers to register locations, initialized to null or the specific registers below.
   uintptr_t* gprs_[kNumberOfXRegisters];
   uint64_t * fprs_[kNumberOfDRegisters];
   // Hold values for sp and pc if they are not located within a stack frame.