tests: backtrace.c accept __libc_do_syscall as first frame symname.

On some architectures (Debian armhl) system calls go through
__libc_do_syscall instead of __kernel_vsyscall. Accept either of
these symbol names for the first backtrace frame.

Signed-off-by: Mark Wielaard <mjw@redhat.com>
diff --git a/tests/backtrace.c b/tests/backtrace.c
index ce0bd17..1d3948e 100644
--- a/tests/backtrace.c
+++ b/tests/backtrace.c
@@ -98,7 +98,8 @@
   {
     case 0:
       if (! reduce_frameno && symname
-	       && strcmp (symname, "__kernel_vsyscall") == 0)
+	       && (strcmp (symname, "__kernel_vsyscall") == 0
+		   || strcmp (symname, "__libc_do_syscall") == 0))
 	reduce_frameno = true;
       else
 	assert (symname && strcmp (symname, "raise") == 0);