checkasm: arm: Offset the location of the stack canary reference

If the maximum number of arguments (currently 15) is changed into an even number, and a function actually takes the full number of arguments, we would have the situation where the checked spot on the stack is at the same place as we store an inverted copy of it.

We already allocate enough space for two values though (for stack alignment purposes, 16 bytes on arm64 and 8 bytes on arm32) so by storing the reference in the upper half of this, the lower half of it works as canary and isn't overwritten.