contrib/check-params-in-docs.py: Ignore target-specific params

contrib/check-params-in-docs.py is a script that checks that all options
reported with gcc --help=params are in gcc/doc/invoke.texi and vice
versa.
gcc/doc/invoke.texi lists target-specific params but gcc --help=params
doesn't.  This meant that the script would mistakenly complain about
parms missing from --help=params.  Previously, the script was just set
to ignore aarch64 and gcn params which solved this issue only for x86.
This patch sets the script to ignore all target-specific params.

contrib/ChangeLog:

	* check-params-in-docs.py: Ignore target specific params.

Signed-off-by: Filip Kastl <fkastl@suse.cz>