kselftest: arm64: fake_sigreturn_misaligned_sp

Add a simple fake_sigreturn testcase which places a valid sigframe on a
non-16 bytes aligned SP. Expects a 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>