More time spent working on printf. I found a case where the fake PC register snuck into the allocator by GCC attempting to hold the low part of a 32-bit value starting in R15.

This was fixed by changing the HARD_REGNO_MODE_OK macro to disallow 32-bit values from being stored in R15. This ensure that only 16-bit values can only be used in the fake PC register, and we already ensure that only happens as part of a function epilogue.