[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

clang/test/Interpreter/execute.cpp

@@ -1,5 +1,6 @@
 // RUN: cat %s | clang-repl | FileCheck %s
 // REQUIRES: host-supports-jit
+// UNSUPPORTED: powerpc64
 
 extern "C" int printf(const char *, ...);
 int i = 42;