spiffs: Make spiffs runnable on host

Makes spiffs component runnable on host. Depends on the host library build
of flash emulator. Includes a basic sanity test of
mounting a volume, opening a file, writing to the file, reading the file,
closing the file and unmounting volume.

components/spiffs/test_spiffs_host/stubs/newlib/include/sys/lock.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#include <time.h>
+
+typedef int _lock_t;
+
+void _lock_acquire(_lock_t *lock);
+void _lock_close(_lock_t *lock);
+void _lock_init(_lock_t *lock);
+void _lock_release(_lock_t *lock);