lib: hash: murmur3: Account for unaligned 32-bit access

The code was casting a byte array to 32-bit words without accounting for
alignment. On some platforms (e.g. Arm Cortex-M with multiple load/store
instructions) this will fault. Fix it by using the UNALIGED_GET() macro
whenever the array is passed unaligned.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>