[compiler-rt] [test] Use %clangxx for tests that use -x c++

When instrumenting C++ code, ubsan ends up referencing
the ubsan_type_hash_* object files, which require linking against
the C++ ABI library. When building with "clang -x c++", the code
is handled as C++, but the compiler still only links as if it was
C.

Change all cases of "%clang -x c++" into "%clangxx -x c++".

This fixes a lot of ubsan tests in mingw mode.

Differential Revision: https://reviews.llvm.org/D147687