RFR: 8178500: Replace usages of round_to and round_down with align_up and align_down

[Stefan Karlsson]

Hi all,

Please review this patch to remove round_to and round_down and replace 
their usages with align_up and align_down.

http://cr.openjdk.java.net/~stefank/8178500/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8178500

The round_to and round_down functions asserted that alignment was a 
power-of-2. I've added asserts to the corresponding align functions.

This patch builds upon all the patches referred to in:
  http://mail.openjdk.java.net/pipermail/hotspot-dev/2017-June/027329.html

Thanks,
StefanK