RFR: 8247910: Improve alignment and power-of-2 utilities using C++14

[Kim Barrett]

> On Sep 23, 2020, at 3:00 AM, Thomas Stüfe <thomas.stuefe at gmail.com> wrote:
> 
> ... Okay I worked around the problem by upgrading to gcc 7. 
> 
> However, should we not make sure that we are buildable on the lowest required compiler level?

gcc5 is the first version of gcc that *claims* to completely support C++14.  That’s the only reason
for setting it as the lower limit; it wasn’t based on data.

Rather than potentially contort code to support what is now a pretty old version, I’d prefer we
moved forward the minimum required version.