[clang-repl] Temporarily disable the execute.cpp test on ppc64.

This test is failing on some builders (see [1]) with the following error:

error: Added modules have incompatible data layouts:
  e-m:e-i64:64-n32:64-S128-v256:256:256-v512:512:512 (module) vs
  E-m:a-i64:64-n32:64-S128-v256:256:256-v512:512:512 (jit)

The JIT layout is correct, but some IR module added to the JIT is using a
little-endian layout instead.

This commit disables the test on ppc64 until we can investigate further and
fix the bug.

[1] https://lab.llvm.org/staging/#/builders/126/builds/371