scripts/get_abi.pl: split label naming from xref logic

Instead of using a ReST compilant label while parsing,
move the label to ReST output. That makes the parsing logic
more generic, allowing it to provide other types of output.

As a side effect, now all files used to generate the output
will be output. We can later add command line arguments to
filter.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>