small replace_sections_syms refactor

Move this code block to a more logical place, outside of the symbol
loop.
section reference replacement for references inside symbols

Currently unbundled section references are only replaced if the start of
the symbol is referenced.  It's also useful to support replacement of
references which point to inside the symbol.
Merge pull request #451 from jpoimboe/sections-syms-fix

 section reference replacement for references inside symbols