dwarf-reader: refactor try_reading_first_ksymtab_entry_using{pre,}_v4_19_format

dwarf-reader: refactor try_reading_first_ksymtab_entry_using{pre,}_v4_19_format

Avoid code duplication and increase maintainebility of these helper
functions. As their only difference was the application of position
relative relocations, consolidate them and add a flag for exactly this
feature.

This is purely stylistic and not changing functionality.

	* src/abg-dwarf-reader.cc(try_reading_first_ksymtab_entry):
	New function to consolidate functionality for
	try_reading_first_ksymtab_entry_using_{pre,}v4_19_format functions.
	(try_reading_first_ksymtab_entry_using_v4_19_format,
	try_reading_first_ksymtab_entry_using_pre_v4_19_format):
	refactor to use try_reading_first_ksymtab_entry

Signed-off-by: Matthias Maennich <maennich@google.com>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>