xtensa: syscall: fix setting up PS for window spilling

We should not perform a straight OR operation on INTLEVEL as
we have no idea what existing PS.INTLEVEL is. Also, to avoid
any interferences, we disable interrupts very early during
syscall entrance. So we can remove the OR operation as
PS.INTLEVEL will still have all interrupts masked. Note that
we do not really need to OR PS_WOE into PS as we currently
only support windowed ABI which must have PS_WOE set in PS
anyway.

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