ARM: diagnose invalid system LDM/STM

The system LDM and STM instructions can't usually writeback to the base
register. The one exception is when an LDM is actually an exception-return
(i.e. contains PC in the register list).

(There's already a test that "ldm sp!, {r0-r3, pc}^" works, which is why there
is no positive test).

rdar://problem/15223374