clover/llvm: Clean up ELF parsing.

clover/llvm: Clean up ELF parsing.

This function was doing three separate things:
 - Initializing and releasing the ELF parsing state (the latter can be
   better done using RAII).
 - Searching for the symbol table in the ELF file.
 - Extraction of kernel symbol offsets from the symbol table.

Split each one into a separate function for clarity and clean up the
result slightly.

Reviewed-by: Serge Martin <edb+mesa@sigluy.net>
Tested-by: Jan Vesely <jan.vesely@rutgers.edu>