fix debug file lookup in bcc_elf_symbol_str
fix debug file lookup in bcc_elf_symbol_str

Logic for looking up debug file in bcc_elf_symbol_str and foreach_sym_core
differ. This commit factors out relevant code from foreach_sym_core and reuses
it in bcc_elf_symbol_str.

brendangregg/bcc,iovisor/bcc,brendangregg/bcc,brendangregg/bcc,tuxology/bcc,iovisor/bcc,tuxology/bcc,brendangregg/bcc,brendangregg/bcc,iovisor/bcc,tuxology/bcc,tuxology/bcc,iovisor/bcc,iovisor/bcc,tuxology/bcc