Merge fixup_section and fixup_symbol_section

Merge fixup_section and fixup_symbol_section

fixup_symbol_section delegates all its work to fixup_section, so merge
the two.

Because there is only a single caller to fixup_symbol_section, we can
also remove some of the introductory logic.  For example, this will
never be called with a NULL objfile any more.

The LOC_BLOCK case can be removed, because such symbols are handled by
the buildsym code now.

Finally, a symbol can only appear in a SEC_ALLOC section, so the loop
is modified to skip sections that do not have this flag set.