BACKPORT: FROMLIST: arm64: Kill 32-bit applications scheduled on 64-bit-only CPUs

Scheduling a 32-bit application on a 64-bit-only CPU is a bad idea.

Ensure that 32-bit applications always take the slow-path when returning
to userspace on a system with mismatched support at EL0, so that we can
avoid trying to run on a 64-bit-only CPU and force a SIGKILL instead.

Signed-off-by: Will Deacon <will@kernel.org>
Bug: 178507149
Link: https://lore.kernel.org/linux-arch/20201208132835.6151-5-will@kernel.org/


[will: Fixed trivial conflict with vendor hook in __switch_to()]
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I5ae90f3fb63499d7016f93d13e32693e26890f92