checkasm: arm/aarch64: Fix the amount of space reserved for stack parameters

Even if MAX_ARGS - 2 (for arm) or MAX_ARGS - 7 (for aarch64) parameters
are passed on the stack to checkasm_checked_call, we actually only
need to store MAX_ARGS - 4 (for arm) or MAX_ARGS - 8 (for aarch64)
parameters on the stack when calling the tested function.

Signed-off-by: Martin Storsjö <martin@martin.st>
