scripts: get_abi.pl: better handle escape chars on what:

The parser for the symbols defined on What: doesn't cover all
chars that need to be scaped, like '{' and '}'. Change the logic
to be more generic, and ensure that the same regex will be used
on both What: and when parsing the cross-references.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/29cb56def89b508fe605bcd2ba74a4376cc08e35.1616668017.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>