[clang] [Fuchsia] Set LIBCXX_ABI_UNSTABLE instead of LIBCXX_ABI_VERSION (PR #189123)

[LLVM Continuous Integration via cfe-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 `clang` at step 11 "ninja check 2".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/76/builds/14312

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

```
Step 11 (ninja check 2) failure: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
...
PASS: LLVM :: tools/llvm-readobj/ELF/file-types.test (108544 of 108570)
PASS: LLVM :: tools/yaml2obj/ELF/section-headers.yaml (108545 of 108570)
PASS: LLVM :: tools/llvm-readobj/ELF/verneed-invalid.test (108546 of 108570)
PASS: SanitizerCommon-msan-powerpc64le-Linux :: sanitizer_coverage_allowlist_ignorelist.cpp (108547 of 108570)
PASS: LLVM :: tools/llvm-readobj/ELF/program-headers.test (108548 of 108570)
PASS: LLVM :: tools/llvm-rc/parser.test (108549 of 108570)
PASS: UBSan-ThreadSanitizer-powerpc64le :: TestCases/ImplicitConversion/unsigned-integer-truncation-ignorelist.c (108550 of 108570)
PASS: LLVM :: tools/llvm-nm/option-X.test (108551 of 108570)
PASS: UBSan-ThreadSanitizer-powerpc64le :: TestCases/ImplicitConversion/unsigned-integer-truncation.c (108552 of 108570)
PASS: LLVM :: FileCheck/dump-input/annotations.txt (108553 of 108570)
command timed out: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=11571.530605

```

</details>

https://github.com/llvm/llvm-project/pull/189123