[scudo] Improve the uses of roundUpTo/roundDownTo/isAligned

The implementations of those functions require the rounding target to be
power-of-two. It's better to add a debugging check to avoid misuse.
Besides, add a general verion of those three to accommadate non
power-of-two cases.

Also change the name to roundUp/roundDown/isAligned

Reviewed By: cferris, cryptoad

Differential Revision: https://reviews.llvm.org/D142658