scripts: get_abi.pl: cleanup ABI cross-reference logic

scripts: get_abi.pl: cleanup ABI cross-reference logic

Right now, the cross-references are generated on a single
step, when doing ReST output.

While this is nice optimization, it prevents auto-creating
cross-references for ABI symbols.

So, split it into a separate logic.

While here, turn on Perl warnings, as it helps to debug
problems inside the script.

Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/dbc97c8c2dfd877921f058134c35b2a8b1f8414b.1604042072.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>