Reorganize ELF writer, add jit::elf namespace

Summary:
There's a lot of changes in this diff, but it's just refactoring, the resulting
ELF object is unchanged.

Switches to writing the section headers first, then the segment headers (aka the
program headers).  The segments will be defined in terms of the sections.

Moves the scattered state of the file (symbol tables, string tables, current
section offset) into an `Object` class that represents the entire file, minus
the code entries themselves.

Also prepare for adding more segments by turning the segment header field into
an array.

Reviewed By: swtaarrs

Differential Revision: D56069175

fbshipit-source-id: cc905d4a3beef78d8733aa0a11c94a51bd40958c