Explicitly specify that ubsan-vtable-checks is x86-64.

This should fix a failure on PowerPC introduced by r287181.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@287185 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit a091fb6ee3b21d859b2e42cbac8d8e51566475bc)
1 file changed