linux-user/signal.c: Put AArch64 frame record in the right place

AArch64 stack frames include a 'frame record' which holds a pointer
to the next frame record in the chain and the LR on entry to the
function. The procedure calling standard doesn't mandate where
exactly this frame record is in the stack frame, but for signal
frames the kernel puts it right at the top. We used to put it
there too, but in commit 7f0f4208 we accidentally put
the "enlarge to the 4K reserved space minimum" check after the
"allow for the frame record" code, rather than before it, with
the effect that the frame record would be inside the reserved
space and immediately after the last used part of it.

Move the frame record back out of the reserved space to where
we used to put it.

This bug shouldn't break any sensible guest code, but test
programs that deliberately look at the internal details
of the signal frame layout will not find what they are
expecting to see.

Fixes: 7f0f4208
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-id: 20180412140222.2096-1-peter.maydell@linaro.org