arch: Add IRQ_OFFSET_NESTED feature

The x86 and xtensa implementations of irq_offload() invoke synchronous
interrupts on the local CPU, and are therefore safe to use from within
an interrupt context.  This is a cheap and portable way to exercise
nested interrupts, which are otherwise highly platform-dependent to
test.  Add a kconfig to signal the capability.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>