__builtin_align_down: Cast value to __uintptr_t in the fallback

[jhb]

Summary

This matches the behavior of the fallbacks for builtin_align_up
and builtin_is_aligned.

[jhb]

I worry that this was intentionally left out to avoid breaking there use of rounddown() on integer types larger than uintptr_t? (E.g. vm_paddr_t on i386)

If that is really a worry, maybe we use _Generic (including the fallback for pre-C11) that maps pointer types to versions that use uintptr_t and otherwise doesn't use the cast?

[arichardson]

Almost certainly an accidental omission