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: ee95fae ("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>
(cherry picked from commit 6400be0)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

linux-user/aarch64/vdso.S

@@ -63,7 +63,11 @@ vdso_syscall __kernel_clock_getres, __NR_clock_getres
  * For now, elide the unwind info for __kernel_rt_sigreturn and rely on
  * the libgcc fallback routine as we have always done.  This requires
  * that the code sequence used be exact.
+ *
+ * Add a nop as a spacer to ensure that unwind does not pick up the
+ * unwind info from the preceding syscall.
  */
+	nop
 __kernel_rt_sigreturn:
 	/* No BTI C insn here -- we arrive via RET. */
 	mov	x8, #__NR_rt_sigreturn