[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 `openmp-offload-sles-build-only` running on `rocm-worker-hw-04-sles` while building `lld` at step 10 "Add check check-lld".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/140/builds/32986

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

```
Step 10 (Add check check-lld) failure: test (failure)
******************** TEST 'lld :: riscv-vendor-relocations.s' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build/bin/llvm-mc -triple riscv32 /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/lld/test/riscv-vendor-relocations.s -filetype=obj -o /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build/tools/lld/test/Output/riscv-vendor-relocations.s.tmp.o
# executed command: /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build/bin/llvm-mc -triple riscv32 /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/lld/test/riscv-vendor-relocations.s -filetype=obj -o /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build/tools/lld/test/Output/riscv-vendor-relocations.s.tmp.o
# .---command stderr------------
# | /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.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