scripts/get_abi.pl: add support for searching for ABI symbols

Change its syntax to allow switching between ReST output mode
and a new search mode, with allows to seek for ABI symbols
using regex.

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