Rewrite the arm mem* builtins without asm and naked functions

[Amanieu]

This also fixes the calling convention on those functions, which should be "aapcs" instead of "C".

[japaric]

Nice! I first tried to implement these function like this but I got an extra bl (or was it bx?) instruction at the end of each routine; that may have been caused by the calling convention I used: "C" instead of "aapcs".