scripts/gdb: delay generation of gdb constants.py

scripts/gdb/linux/constants.py is never used in the kernel build
process. There is no good reason to create it so early.

Get it out of the 'prepare' stage.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>