[yaml2obj][ELF] Add the section name -> section index map to State.

One of the key things that the YAML format abstracts over is the use of
section numbers for referencing sections. Instead, textual section names
are used, which yaml2obj then translates into appropriate section
numbers. (Technically ELF doesn't care about section names (only section
numbers), but since this is a testing tool, readability counts).

This simplifies using section names as symbolic references in various
parts of the code. An upcoming commit will use this to allow symbols to
reference sections.
[yaml2obj][ELF] Allow symbols to reference sections.
[yaml2obj][ELF] Add support for st_value and st_size.

After this patch, the ELF file produced by
`yaml2obj-elf-symbol-basic.yaml`, when linked and executed on x86_64
(under SysV ABI, obviously; I tested on Linux), produces a working
executable that goes into an infinite loop!