[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-solaris11-sparcv9` running on `solaris11-sparcv9` while building `llvm` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/13/builds/4179

<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-opcode-name-FADD_D.s' FAILED ********************
Exit Code: 2

Command Output (stderr):
--
RUN: at line 1: /opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/stage1/bin/llvm-exegesis -mtriple=riscv64-unknown-linux-gnu --mcpu=generic -mode=latency --benchmark-phase=assemble-measured-code -mattr=+d -opcode-name=FADD_D | /opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/stage1/bin/FileCheck /opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/llvm/llvm/test/tools/llvm-exegesis/RISCV/latency-by-opcode-name-FADD_D.s
+ /opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/stage1/bin/llvm-exegesis -mtriple=riscv64-unknown-linux-gnu --mcpu=generic -mode=latency --benchmark-phase=assemble-measured-code -mattr=+d -opcode-name=FADD_D
+ /opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/stage1/bin/FileCheck /opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/llvm/llvm/test/tools/llvm-exegesis/RISCV/latency-by-opcode-name-FADD_D.s
llvm-exegesis error: no LLVM target for triple riscv64-unknown-linux-gnu
FileCheck error: '<stdin>' is empty.
FileCheck command line:  /opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/stage1/bin/FileCheck /opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/llvm/llvm/test/tools/llvm-exegesis/RISCV/latency-by-opcode-name-FADD_D.s

--

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


```

</details>

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