abidw: add option to only emit file names (--short-locs)

abidw: add option to only emit file names (--short-locs)

Various emitted directories contain machine specific information and
therefore break reproducibility of abidw's output across different
build paths.

Hence introduce --short-locs to only emit file names.

Thanks to earlier changes, adding an option boils down to adding it to
set_opts and to the write_context along with some auxiliary functions
for setting and getting.

	* include/abg-writer.h (set_short_locs): Declare new function.
	(set_common_options): Use it.
	set_opts
	* src/abg-writer.cc (write_context::m_short_locs): New data
	member.
	(write_context::write_context): Initialize it.
	(write_context::{g,s}et_short_locs): Define new accessors.
	(write_location, write_translation_unit, write_corpus): Honour the
	new write_context::get_short_locs property.
	(set_short_locs): Define new function.
	* tools/abidw.cc (options::short_locs): New data member.
	(display_usage): Help string for the new --no-show-locs option.
	(parse_command_line): Parse the new --no-show-locs option.

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