linux-user/aarch64: Add padding before __kernel_rt_sigreturn

linux-user/aarch64: Add padding before __kernel_rt_sigreturn

Without this padding, an unwind through the signal handler
will pick up the unwind info for the preceding syscall.

This fixes gcc's 30_threads/thread/native_handle/cancel.cc.

Cc: qemu-stable@nongnu.org
Fixes: ee95fae075c6 ("linux-user/aarch64: Add vdso")
Resolves: https://linaro.atlassian.net/browse/GNU-974
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240202034427.504686-1-richard.henderson@linaro.org>