Add a label at the end of the code to indicate the literal pool.

This makes objdump disassembly of the code a bit clearer, by separating
the constants from code following the last label.

firmware/hackrf_usb/sgpio_m0.s

@@ -239,3 +239,6 @@ done:
 	str offset, [state, #OFFSET]                    // state.offset = offset                // 2
 
 	b loop                                          // goto loop                            // 3
+
+// The linker will put a literal pool here, so add a label for clearer objdump output:
+constants: