lib: Handle traps when doing unpriv load/store in get_insn()

We can get a page/access trap when doing unpriv load/store in
get_insn() function because on a SMP system Linux swapper running
on HART A can unmap pages from page table used by HART B.

To tackle this we extend get_insn() implementation so that if
we get trap in get_insn() then we redirect it to S-mode as fetch
page/access fault.

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