arch: arm: error: fix ARMv6-M assembly for Z_ARCH_EXCEPT

As we are allowed to pass any integer value as as software
fatal exception reason, we need to fix the inline assembly
for ARMv6-M, to accept large immediate offsets. We do this
by changing the way we write the exception reason to R0.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>