Return the alignment required from c64_valid_cap_range

Return the alignment required from c64_valid_cap_range

We were specifying section alignment requirements based on the alignment
that the section base happened to have.  This sometimes resulted in very
strange alignment requests that were much greater than actually
required.
That is not usually a problem, but it does give unnecessary padding upon
re-adjustments due to changing the PCC bounds after individual sections
have been padded.

This patch adds an interface such that we return the alignment actually
required for exact capability bounds from c64_valid_cap_range.  We then
use that alignment as our alignment requirement on the sections which
have a section-sized symbol associated with them.