[AVR32] Fix NMI handler

Fix a problem with the NMI handler entry code related to the NMI handler
sharing some code with the exception handlers. This is not a good idea
because the RSR and RAR registers are not the same, and the NMI handler
runs with interrupts masked the whole time so there's no need to check
for pending work.

Open-code the low-level NMI handling logic instead so that the pt_regs
layout is actually correct when the higher-level handler is called.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>