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

[Min-Yih Hsu via llvm-commits]

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+# RUN: llvm-exegesis -mode=latency -mtriple=riscv64-unknown-linux-gnu --benchmark-phase=assemble-measured-code -opcode-name=LD | FileCheck --check-prefix=TEST1 %s
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mshockwave wrote:

nit: usually all RUN lines would be put at the very top of the file. Also, could you use the opcode you're testing as FileCheck prefix rather than `TESTX`?

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