Prologue/Epilogue: No need to modify SP in leaf functions

[mbitsnbites]

In leaf functions there is no need to update the SP register (i.e. subtract in the prologue and add in the epilogue).

So this:

    add     sp, sp, #-8
    stw     r17, [sp, #4]
    stw     r16, [sp, #0]
    ...
    ldw     r16, [sp, #0]
    ldw     r17, [sp, #4]
    add     sp, sp, #8
    ret

...could be turned into this:

    stw     r17, [sp, #-4]
    stw     r16, [sp, #-8]
    ...
    ldw     r16, [sp, #-8]
    ldw     r17, [sp, #-4]
    ret