testsuite: arm: Mark pr81812.C as xfail for thumb1

Test fails for Cortex-M0 with:

.../pr81812.C:6:8: error: generic thunk code fails for method 'virtual void ChildNode::_ZTv0_n12_NK9ChildNode5errorEz(...) const' which uses '...'

According to PR108277, it's expected that thumb1 targets does not
support empty virtual functions with ellipsis.

gcc/testsuite/ChangeLog:

	* g++.dg/torture/pr81812.C: Add xfail for thumb1.

Signed-off-by: Torbjörn SVENSSON <[email protected]>

Author: Torbjorn-Svensson

gcc/testsuite/g++.dg/torture/pr81812.C

@@ -1,3 +1,5 @@
+// { dg-xfail-if "PR108277" { arm_thumb1 } }
+
 struct Error {
   virtual void error(... ) const;
 };