Use the "retl" instead of "ret" instruction alias, since we
do not allocate a register window in this function.

Fix the offset to the first stacked parameter, which lies
beyond the register window save area.

Fixes: 95c021da ("linux-user/host/sparc64: Add safe-syscall.inc.S")
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>