scripts: build: gen_isr_tables: change naming of bitmask variables

Rename the bitmask variables from `*_LVL_INTERRUPTS` to
`INTERRUPT_LVL_BITMASK[]` array to be consistent with
`INTERRUPT_BITS`, making it easier to loop over the bitmasks.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>