[llvm] [CAS] Default to only build OnDiskCAS on 64 bit machine (PR #158737)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-ppc64le-linux-multistage` running on `ppc64le-clang-multistage-test` while building `llvm` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/76/builds/12885

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 5 (ninja check 1) failure: 1200 seconds without output running [b'ninja', b'check-all', b'check-runtimes'], attempting to kill
...
[79/85] Generating POWERPC64LELinuxConfig/Asan-powerpc64le-calls-Noinst-Test
[80/85] Generating ASAN_INST_TEST_OBJECTS.gtest-all.cc.powerpc64le-calls.o
[81/85] Generating POWERPC64LELinuxDynamicConfig/Asan-powerpc64le-calls-Dynamic-Test
[82/85] Generating POWERPC64LELinuxConfig/Asan-powerpc64le-calls-Test
[83/85] Generating ASAN_INST_TEST_OBJECTS.gtest-all.cc.powerpc64le-inline.o
[84/85] Generating POWERPC64LELinuxDynamicConfig/Asan-powerpc64le-inline-Dynamic-Test
[85/85] Generating POWERPC64LELinuxConfig/Asan-powerpc64le-inline-Test
[1035/1257] No install step for 'runtimes'
[1036/1257] cd /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/stage1/runtimes/runtimes-bins && /home/buildbots/llvm-external-buildbots/cmake-3.31.2/bin/cmake --build /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/stage1/runtimes/runtimes-bins/ --target check-runtimes --config Release
[0/1] Running runtimes regression tests
command timed out: 1200 seconds without output running [b'ninja', b'check-all', b'check-runtimes'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1474.768851

```

</details>

https://github.com/llvm/llvm-project/pull/158737