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

[via llvm-commits]

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+RUN: llvm-exegesis -mode=latency --benchmark-phase=assemble-measured-code -opcode-name=LB -mtriple=riscv64-unknown-linux-gnu
+
+CHECK: Warning: Pre-assigned register prevented usage of self-aliasing strategy.
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AnastasiyaChernikova wrote:

This test was used previously, during implementing, now it is not used anymore, removed it, thanks for the catch.

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