lib: Use MTINST CSR in misaligned load/store emulation

We should use MTINST CSR in misaligned load/store emulation whenever
possible to avoid unpriv read in getting trapped instruction. This will
improve preformance on HW having proper implementation of MTINST CSR.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>