kselftest: arm64: mangle_pstate_invalid_mode_el[123][ht]

Add 6 simple mangle testcases that mess with the ucontext_t from within
the signal handler, trying to toggle PSTATE mode bits to trick the system
into switching to EL1/EL2/EL3 using both SP_EL0(t) and SP_ELx(h).
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>