Add constructor to spinlock (in C++)

Added constructor to the "spinlock" type (#include <osv/mutex.h>) when
used from C++. This makes it easier to use this type from C++.

Marked constructor as "constexpr" to allow the object to be constructed
in compile time (e.g., a global spinlock variable can be initialized in
bss). Thanks Avi for this trick!

include/osv/mutex.h

@@ -14,6 +14,7 @@ typedef struct spinlock {
     bool _lock;
 #ifdef __cplusplus
     // additional convenience methods for C++
+    inline constexpr spinlock() : _lock(false) { }
     inline void lock();
     inline void unlock();
 #endif