gen_compile_commands: make -o option independent of -d option

Change the -o option independent of the -d option, which is I think
clearer behavior. Some people may like to use -d to specify a separate
output directory, but still output the compile_commands.py in the
source directory (unless the source tree is read-only) because it is
the default location Clang Tools search for the compilation database.

Also, move the default parameter to the default= argument of the
.add_argument().

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