firmware: fw_base: Improve exception stack setup in trap handler

Currently, the low-level trap handler (i.e. _trap_handler()) uses
branch instructions to conditionally setup exception stack based
on which mode trap occured.

This patch implements exception stack setup using xor instructions
which is faster with same number of instructions due to lack of
branch instructions.

The new exception stack setup approach can be best described by the
following pseudocode:

 Came_From_M_Mode = ((MSTATUS.MPP < PRV_M) ? 1 : 0) - 1;
 Exception_Stack = TP ^ (Came_From_M_Mode & (SP ^ TP))

 Came_From_M_Mode = 0    ==>    Exception_Stack = TP
 Came_From_M_Mode = -1   ==>    Exception_Stack = SP

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>