[lld] [llvm] [llvm][LLD][COFF] Add fat-lto-object support for COFF targets (PR #165529)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-aarch64-darwin` running on `doug-worker-5` while building `lld,llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/190/builds/33051

<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/fatlto/fatlto-invalid.s' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/llvm-mc -filetype=obj -triple=i386-pc-win32 /Users/buildbot/buildbot-root2/aarch64-darwin/llvm-project/lld/test/COFF/fatlto/fatlto-invalid.s -o /Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/tools/lld/test/COFF/fatlto/Output/fatlto-invalid.s.tmp
# executed command: /Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/llvm-mc -filetype=obj -triple=i386-pc-win32 /Users/buildbot/buildbot-root2/aarch64-darwin/llvm-project/lld/test/COFF/fatlto/fatlto-invalid.s -o /Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/tools/lld/test/COFF/fatlto/Output/fatlto-invalid.s.tmp
# .---command stderr------------
# | /Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/llvm-mc: error: unable to get target for 'i386-pc-windows-msvc', see --version and --triple.
# `-----------------------------
# error: command failed with exit status: 1

--

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


```

</details>

https://github.com/llvm/llvm-project/pull/165529