Add a stub pthread header.

The header implements only the pthread rwlock interface, which
never actually locks.

This is necessary to build libunwind.

software/include/base/pthread.h

@@ -0,0 +1,27 @@
+#ifndef __PTHREAD_H
+#define __PTHREAD_H
+
+typedef int pthread_rwlock_t;
+
+#define PTHREAD_RWLOCK_INITIALIZER 0
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+inline int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock)
+  { return 0; }
+inline int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock)
+  { return 0; }
+inline int pthread_rwlock_wrlock(pthread_rwlock_t *rwlock)
+  { return 0; }
+inline int pthread_rwlock_trywrlock(pthread_rwlock_t *rwlock)
+  { return 0; }
+int pthread_rwlock_unlock(pthread_rwlock_t *rwlock)
+  { return 0; }
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* __PTHREAD_H */