add naked attribute for empty function
add naked attribute for empty function

This is required on ARM to prevent the complier from emitting
prolog / epilog sequences that would save the clobbered register
to the stack. Prior to this change, compiling emitted:

    main.c: In function '_start':
    main.c:6:6: warning: stack usage is 4 bytes [-Wstack-usage=]

after this change, the warning is gone.

tock/libtock-c,tock/libtock-c,tock/libtock-c