linux-user/s390x: Use the guest pointer for the sigreturn stub

When setting up the pointer for the sigreturn stub in the return
address register (r14) we currently use the host frame address instead
of the guest frame address.

Note: This only caused problems if Qemu has been built with
--disable-pie (as it is in distros nowadays). Otherwise guest_base
defaults to 0 hiding the actual problem.

Signed-off-by: Andreas Krebbel <krebbel@linux.ibm.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210324185128.63971-1-krebbel@linux.ibm.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>