Synthetic symbol table for 64-bit PowerPC

64-bit PowerPC contains [function descriptor section .opd](http://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi.html#FUNC-DES). The real symbol address is not in the symbol table. To get to it, `.opd` needs to be used. GNU tools solve this by introducing _synthetic symbol table_. It looks like an ordinary symbol table, but symbol names are prefixed with `.` (dot), and values reference real addresses/offsets.

We will need this to create statically linked function patterns, and eventual decompilation of 64-bit PowerPC.

Start by reading the linked 64-bit PowerPC ELF ABI description and comparing how GNU tools expose synthetic symbols for the .opd section. Done means RetDec can represent dot-prefixed synthetic symbols whose values refer to the real function addresses or offsets, enabling statically linked function patterns.

Written by the indexing model from the issue text.