[llvm] [Exegesis][RISCV] Add RISCV support for llvm-exegesis (PR #89047)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-nvptx-nvidia-win` running on `as-builder-8` while building `llvm` at step 7 "test-build-unified-tree-check-llvm".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/54/builds/4885

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

```
Step 7 (test-build-unified-tree-check-llvm) failure: test (failure)
******************** TEST 'LLVM :: tools/llvm-exegesis/RISCV/latency-by-extension-C.s' FAILED ********************
Exit Code: 2

Command Output (stdout):
--
# RUN: at line 1
c:\buildbot\as-builder-8\llvm-nvptx-nvidia-win\build\bin\llvm-exegesis.exe -mode=latency -mtriple=riscv64-unknown-linux-gnu --mcpu=generic --benchmark-phase=assemble-measured-code -opcode-name=C_ADDI -mattr=+c | c:\buildbot\as-builder-8\llvm-nvptx-nvidia-win\build\bin\filecheck.exe --check-prefix=C_ADDI C:\buildbot\as-builder-8\llvm-nvptx-nvidia-win\llvm-project\llvm\test\tools\llvm-exegesis\RISCV\latency-by-extension-C.s
# executed command: 'c:\buildbot\as-builder-8\llvm-nvptx-nvidia-win\build\bin\llvm-exegesis.exe' -mode=latency -mtriple=riscv64-unknown-linux-gnu --mcpu=generic --benchmark-phase=assemble-measured-code -opcode-name=C_ADDI -mattr=+c
# .---command stderr------------
# | llvm-exegesis error: no LLVM target for triple riscv64-unknown-linux-gnu
# `-----------------------------
# error: command failed with exit status: 1
# executed command: 'c:\buildbot\as-builder-8\llvm-nvptx-nvidia-win\build\bin\filecheck.exe' --check-prefix=C_ADDI 'C:\buildbot\as-builder-8\llvm-nvptx-nvidia-win\llvm-project\llvm\test\tools\llvm-exegesis\RISCV\latency-by-extension-C.s'
# .---command stderr------------
# | FileCheck error: '<stdin>' is empty.
# | FileCheck command line:  c:\buildbot\as-builder-8\llvm-nvptx-nvidia-win\build\bin\filecheck.exe --check-prefix=C_ADDI C:\buildbot\as-builder-8\llvm-nvptx-nvidia-win\llvm-project\llvm\test\tools\llvm-exegesis\RISCV\latency-by-extension-C.s
# `-----------------------------
# error: command failed with exit status: 2

--

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


```

</details>

https://github.com/llvm/llvm-project/pull/89047