xtensa: userspace: use ADDX4 to calculate syscall table index

When looking for jump address in the syscall table, we need to
multiply the syscall ID by 4 before adding the address offset
of the beginning of the table. This is due to the jump address
being 32-bit (4 bytes). Instead of using two instructions to
shift the ID by 4 first and then the addition, we can use one
ADDX4 instruction to achieve the same result.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>