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 <torbjorn.svensson@foss.st.com>