[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-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/38612

<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 :: riscv-vendor-relocations.s' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/llvm-mc -triple riscv32 /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/lld/test/riscv-vendor-relocations.s -filetype=obj -o /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/tools/lld/test/Output/riscv-vendor-relocations.s.tmp.o
# executed command: /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/llvm-mc -triple riscv32 /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/lld/test/riscv-vendor-relocations.s -filetype=obj -o /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/tools/lld/test/Output/riscv-vendor-relocations.s.tmp.o
# .---command stderr------------
# | /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/llvm-mc: 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