gen_compile_commands: use choices for --log_levels option

Use 'choices' to check if the given parameter is valid.

I also simplified the help message because, with 'choices', --help
shows the list of valid parameters:

  --log_level {DEBUG,INFO,WARNING,ERROR,CRITICAL}

I started the help message with a lower case, "the level of log ..."
in order to be consistent with the -h option:

  -h, --help            show this help message and exit

The message "show this help ..." comes from the ArgumentParser library
code, and I do not know how to change it. So, I changed our code.

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