Fix exception address alignment test for EHABI

This test fails on ARM bare-metal targets because it assumes the Itanium ABI,
whereas EHABI requires the exception address to be 8-byte aligned.

I was a bit puzzled at first because this should've failed on the public
arm-linux builder too. I think the reason it passes there is because we don't
include libunwind headers in the include path when running the libcxxabi tests,
so the system unwind.h gets picked up.

Reviewers: rengolin, EricWF
Differential revision: https://reviews.llvm.org/D31178

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@299435 91177308-0d34-0410-b5e6-96231b3b80d8