Convert output symbol table back to a table

It looks like at some point the output symbol
was converted from an actual table of SYMREC
structs to an in-memory copy of a COFF/a.out
symbol table. This entailed relying on the
assumption that all symbols had an entry in the
string table when using OSTLookup()'s return
value as anything other than a boolean value,
as is done in the relocation procesing logic.

In preparation for adding support for debug
symbols, which often have no string table entry,
revert to using an intermediate output symbol
table representation and serializing it one symbol
at a time when writing the output file. This
simplifies various code paths, but potentially
slows down writing COFF symbol tables to disk.
Fortunately, this table is not written with
default options, and is rather small unless
using debug symbols, so this shouldn't
significantly affect the runtime of most
existing use cases.