kselftest: arm64: fake_sigreturn_bad_size_for_magic0

Add a simple fake_sigreturn testcase which builds a ucontext_t with a
badly sized terminator record and place it onto the stack.
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>