[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 `clang-aarch64-quick` running on `linaro-clang-aarch64-quick` while building `llvm` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/65/builds/9564

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

```
Step 5 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'LLVM :: tools/llvm-exegesis/RISCV/latency-by-extension-A.s' FAILED ********************
Exit Code: 2

Command Output (stderr):
--
RUN: at line 1: /home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/bin/llvm-exegesis -mode=latency -mtriple=riscv64-unknown-linux-gnu --mcpu=generic --benchmark-phase=assemble-measured-code -opcode-name=AMOAND_D -mattr="+a" | /home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/bin/FileCheck --check-prefix=AMOAND_D /home/tcwg-buildbot/worker/clang-aarch64-quick/llvm/llvm/test/tools/llvm-exegesis/RISCV/latency-by-extension-A.s
+ /home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/bin/llvm-exegesis -mode=latency -mtriple=riscv64-unknown-linux-gnu --mcpu=generic --benchmark-phase=assemble-measured-code -opcode-name=AMOAND_D -mattr=+a
+ /home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/bin/FileCheck --check-prefix=AMOAND_D /home/tcwg-buildbot/worker/clang-aarch64-quick/llvm/llvm/test/tools/llvm-exegesis/RISCV/latency-by-extension-A.s
llvm-exegesis error: no LLVM target for triple riscv64-unknown-linux-gnu
FileCheck error: '<stdin>' is empty.
FileCheck command line:  /home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/bin/FileCheck --check-prefix=AMOAND_D /home/tcwg-buildbot/worker/clang-aarch64-quick/llvm/llvm/test/tools/llvm-exegesis/RISCV/latency-by-extension-A.s

--

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


```

</details>

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