Interrupt priority assert improvements for CM3/4/7 (#602)

Interrupt priority assert improvements for CM3/4/7 (#602)

* Interrupt priority assert improvements for CM3/4/7

In the ARM_CM3, ARM_CM4, and ARM_CM7 ports, change the assertion that
`configMAX_SYSCALL_INTERRUPT_PRIORITY` is nonzero to account for the
number of priority bits implemented by the hardware.

Change these ports to also use the lowest priority for PendSV and
SysTick, ignoring `configKERNEL_INTERRUPT_PRIORITY`.

* Remove not needed configKERNEL_INTERRUPT_PRIORITY define

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>
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Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>
Co-authored-by: Gaurav-Aggarwal-AWS <33462878+aggarg@users.noreply.github.com>
Co-authored-by: Gaurav Aggarwal <aggarg@amazon.com>