arch: Add support for dynamic shared interrupts

This works by overwriting z_isr_install()'s definition
(possible since the symbol is now weak) with our own definiton.

Whenever trying to register a new ISR/arg pair, z_isr_install()
will check to see if the interrupt line is already in use. If it's
not then it will not share the interrupt and will work exactly
as the old z_isr_install(), meaning it will just write the new
ISR/arg pair to _sw_isr_table.

If the interrupt line is already being used by an ISR/arg pair
then that line will become shared, meaning we'll overwrite
_sw_isr_table with our own (z_shared_isr, z_shared_sw_isr_table[irq])
pair.

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>