[lld] [lld] Add infrastructure for handling RISCV vendor-specific relocations. (PR #159987)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-sie-win` running on `sie-win-worker` while building `lld` at step 4 "clean-build-dir".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/46/builds/25355

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

```
Step 4 (clean-build-dir) failure: Delete failed. (failure)
Step 7 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'lld :: riscv-vendor-relocations.s' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
z:\b\llvm-clang-x86_64-sie-win\build\bin\llvm-mc.exe -triple riscv32 Z:\b\llvm-clang-x86_64-sie-win\llvm-project\lld\test\riscv-vendor-relocations.s -filetype=obj -o Z:\b\llvm-clang-x86_64-sie-win\build\tools\lld\test\Output\riscv-vendor-relocations.s.tmp.o
# executed command: 'z:\b\llvm-clang-x86_64-sie-win\build\bin\llvm-mc.exe' -triple riscv32 'Z:\b\llvm-clang-x86_64-sie-win\llvm-project\lld\test\riscv-vendor-relocations.s' -filetype=obj -o 'Z:\b\llvm-clang-x86_64-sie-win\build\tools\lld\test\Output\riscv-vendor-relocations.s.tmp.o'
# .---command stderr------------
# | z:\b\llvm-clang-x86_64-sie-win\build\bin\llvm-mc.exe: error: unable to get target for 'riscv32', see --version and --triple.
# `-----------------------------
# error: command failed with exit status: 1

--

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


```

</details>

https://github.com/llvm/llvm-project/pull/159987