interrupts: Do not assert on IRQ enable status for ISR install on GIC

The current `z_isr_install` implementation asserts that the IRQ to
which the ISR will be installed must be disabled.

This commit disables that assertion for the ARM GIC because the SGI-
type IRQs can never be disabled as per the specifications and this
causes the assertion to fail for them.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>