Add --no-architecture option to abidw

Add --no-architecture option to abidw

For portability of the regression tests of libabigail, we need abidw
to avoid emitting information about the architecture of the ELF file
it's looking at.  This patch thus adds a new --no-architecture option
to abidw for that purpose.

	* tools/abidw.cc (options::write_architecture): New flag.
	(options::options): Initialize it.
	(display_usage): Add usage string for the new --no-architecture
	option.
	(parse_command): Parse the new --no-architecture command line
	option.
	(main): Ignore the architecture name if required by the user.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>