A previous PR changed a whole lot of min/maxes to std::min/std::max
but made a mistake here and used a templated min which cast it's
arguments to unsigned instead of casting return value.
This resulted in glitchy artifacts in bright areas (See issue 7439)
I rewrote the code to use a proper clamping function so it's cleaner
to read.
Our defines were never clear between what meant 64bit or x86_64
This makes a clear cut between bitness and architecture.
This commit also has the side effect of bringing up aarch64 compiling support.
Scott Mansell