Where to put ALIGN_UP and ALIGN_DOWN macros?

[Carlos O'Donell]

While working on BZ#11787, and dealing with the stack allocation I've
written ALIGN_DOWN and ALIGN_UP macros. I'd like to start using those
throughout the code when we need to align values in C. The macros
avoid the inconsistency we have in rounding code as I documented here:
http://sources.redhat.com/ml/libc-alpha/2012-04/msg00006.html

If we accept their use and inclusion, where should we put them?

New header specific to alignment macros?

Repurpose another header like libc-internal?

Cheers,
Carlos.