kselftest: arm64: mangle_pstate_invalid_daif_bits

Add a simple mangle testcase which messes with the ucontext_t from within
the signal handler, trying to set PSTATE DAIF bits to an invalid value
(masking everything). Expects SIGSEGV on test PASS.

Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>