linux-user/arm: Implement setup_sigtramp

linux-user/arm: Implement setup_sigtramp

Mirror what the kernel does in arch/arm/kernel/signal.h,
using the old sigframe struct in the rt sigframe struct.

Update the trampoline code to match the kernel: this uses
sp-relative accesses rather than pc-relative.

Copy the code into frame->retcode from the trampoline page.
This minimises the different cases wrt arm vs thumb vs fdpic.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20210929130553.121567-6-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>