Use default section indexes in fixup_symbol_section

Use default section indexes in fixup_symbol_section

If fixup_section does not find a matching section, it arbitrarily
chooses the first one.  However, it seems better to make this default
depend on the type of the symbol -- i.e., default data symbols to
.data and text symbols to .text.

I've also made fixup_section static, as it only has one caller.