Disable instrprof-merging.cpp on powerpc64 (D69471 follow up)

An execution count goes missing for a constructor, this needs
investigation:

http://lab.llvm.org:8011/builders/clang-ppc64be-linux/builds/45132/

```
/home/buildbots/ppc64be-clang-test/clang-ppc64be/llvm/compiler-rt/test/profile/instrprof-merging.cpp:28:16:
error: V1: expected string not found in input
 A() {} // V1: [[@LINE]]{{ *}}|{{ *}}1

<stdin>:28:32: note: possible intended match here
 28| | A() {} // V1: [[@LINE]]{{ *}}|{{ *}}1
```