gen_isr_tables: fix check for CONFIG_3RD_LEVEL_INTERRUPTS

If CONFIG_3RD_LEVEL_INTERRUPTS is not enabled then we see the following
error during sw_isr_table generation:
Traceback (most recent call last):
  File "zephyr/arch/common/gen_isr_tables.py", line 291, in <module>
    main()
  File "zephyr/arch/common/gen_isr_tables.py", line 199, in main
    if syms["CONFIG_3RD_LEVEL_INTERRUPTS"]:
KeyError: 'CONFIG_3RD_LEVEL_INTERRUPTS'
ninja: build stopped: subcommand failed.

Fix the logic to look for the symbol instead of referencing it.

Signed-off-by: Michael Scott <mike@foundries.io>