builtins: correct __umodsi3, __udivsi3 on ARM

When building the builtins for a modern CPU (idiv support), __umodsi3 was
completely incorrect as it would behave as __udivmosi3, which takes a tertiary
parameter which is a pointer.

__udivsi3 was also incorrect, returning the remainder in r1.  Although this
would not result in any crash or invalid behaviour as r1 is a caller saved
register in AAPCS, this is unnecessary.  Simply perform the division ignoring
the remainder.
builtins: add signature to some assembly routines

Add a helpful description and a signature for the functions implemented in
assembly for the integral math routines.  NFC.