thermosphere: fix bugs:

- missing barriers after setting elr/spsr
- .text.start* matching .text.startup (which contains main, thanks @fincs)

thermosphere/src/main.c

@@ -0,0 +1,7 @@
+#include "utils.h"
+
+int main(void)
+{
+    // Setup stuff
+    return 0;
+}