include: Make mstatus parameter optional for get_insn()

The mstatus parameter of get_insn() is used to return MSTATUS CSR
value which get_insn() saw. Most of the get_insn() callers don't
use the value returned in mstatus so this patch makes mstatus
parameter optional for get_insn().

Signed-off-by: Anup Patel <anup.patel@wdc.com>