fix main return for msp430

mspgcc handles main specially, this introduces code to make main
behave normally

cpu/msp430-common/cpu.c

@@ -39,6 +39,14 @@ void cpu_switch_context_exit(void)
     __restore_context();
 }
 
+/**
+ * mspgcc handles main specially - it does not return but falls
+ * through to section .fini9.
+ * To "fix" this, we put a return in section .fini9 to make main
+ * behave like a regular function. This enables a common
+ * thread_stack_init behavior. */
+__attribute__((section (".fini9"))) void __main_epilogue(void) { __asm__("ret"); }
+
 //----------------------------------------------------------------------------
 // Processor specific routine - here for MSP
 //----------------------------------------------------------------------------