target-arm: Ignore low bit of PC in M-profile exception return

target-arm: Ignore low bit of PC in M-profile exception return

For the ARM M-profile cores, exception return pops various registers
including the PC from the stack. The architecture defines that if the
lowest bit in the new PC value is set (ie the PC is not halfword
aligned) then behaviour is UNPREDICTABLE. In practice hardware
implementations seem to simply ignore the low bit, and some buggy
RTOSes incorrectly rely on this. QEMU's behaviour was architecturally
permitted, but bringing QEMU into line with the hardware behaviour
allows more guest code to run. We log the situation as a guest error.

This was reported as LP:1428657.

Reported-by: Anders Esbensen <anders@lyes.dk>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>