accel/tcg/user-exec: Don't parse aarch64 insns to test for read vs write

In cpu_signal_handler() for aarch64 hosts, currently we parse
the faulting instruction to see if it is a load or a store.
Since the 3.16 kernel (~2014), the kernel has provided us with
the syndrome register for a fault, which includes the WnR bit.
Use this instead if it is present, only falling back to
instruction parsing if not.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
I wanted to say "everybody surely has at least a 3.16
kernel for aarch64 machines" and delete the fallback code,
but it turns out that the gcc compile farm box has 3.13.0...
1 file changed