gen_compile_commands: move directory walk to a generator function

Currently, this script walks under the specified directory (default to
the current directory), then parses all .cmd files found.

Split it into a separate helper function because the next commit will
add more helpers to pick up .cmd files associated with given file(s).

There is no point to build and return a huge list at once. I used a
generator so it works in the for-loop with less memory.

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