'b' can't jump very far on Cortex-M0 and will cause linker issues when
isr_wrapper and _IntExit are placed far away from each other.
To resolve this we use the 'bx' instruction, as it can jump much
further. Using 'bx' is not dangerous because we are jumping to thumb
mode code.
Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
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