When delivering a signal to a kernel task, or in CONFIG_BUILD_FLAT with MMU
enabled, the REG_SCTLR_EL1 needs to be stored for exception return.
Otherwise 0 is restored to the register at exception return, MMU is switched off
and the system crashes.
Signed-off-by: Jukka Laitinen <jukka.laitinen@tii.ae>