[lld] [LLD][COFF] Add support for -includeoptional on ARM64X (PR #126300)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-sie-ubuntu-fast` running on `sie-linux-worker` while building `lld` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/144/builds/17798

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

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'lld :: COFF/arm64x-includeoptional.s' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 3: /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/llvm-mc -filetype=obj -triple=aarch64-windows /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/lld/test/COFF/arm64x-includeoptional.s -o /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/tools/lld/test/COFF/Output/arm64x-includeoptional.s.tmp.arm64.obj
+ /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/llvm-mc -filetype=obj -triple=aarch64-windows /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/lld/test/COFF/arm64x-includeoptional.s -o /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/tools/lld/test/COFF/Output/arm64x-includeoptional.s.tmp.arm64.obj
/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/llvm-mc: error: unable to get target for 'aarch64-unknown-windows-msvc', see --version and --triple.
--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/126300