macho: do not assume every object has a symtab

For example, building stage2 requires an empty `empty.cc` source file
compiling which generates a valid translation unit with no symtab/strtab.
In this case, we cannot simply assume that every translation unit will
have a valid symtab; instead, we cautiously default the input symtab
and strtab fields to optional `null` to signal symtab's presence or its lack of.
In case the symtab is not present, we catch this fact when splitting
input sections into subsections and create a synthetic symbol per every
suitable section.

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