target/arm/arm-semi: Capture errno in softmmu version of set_swi_errno()

target/arm/arm-semi: Capture errno in softmmu version of set_swi_errno()

The set_swi_errno() function is called to capture the errno
from a host system call, so that we can return -1 from the
semihosting function and later allow the guest to get a more
specific error code with the SYS_ERRNO function. It comes in
two versions, one for user-only and one for softmmu. We forgot
to capture the errno in the softmmu version; fix the error.

(Semihosting calls directed to gdb are unaffected because
they go through a different code path that captures the
error return from the gdbstub call in arm_semi_cb() or
arm_semi_flen_cb().)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190916141544.17540-2-peter.maydell@linaro.org