gen_compile_commands: support *.o, *.a, modules.order in positional argument

This script currently searches the specified directory for .cmd files.
One drawback is it may contain stale .cmd files after you rebuild the
kernel several times without 'make clean'.

This commit supports *.o, *.a, and modules.order as positional
parameters. If such files are given, they are parsed to collect
associated .cmd files. I added a generator helper for each of them.

This feature is useful to get the list of active .cmd files from the
last build, and will be used by the next commit to wire up the
compile_commands.json rule to the Makefile.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>