[llvm] Reland (#120419):[Exegesis][RISCV] Add RISCV support for llvm-exegesis (PR #120467)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

@dybv-sc Thanks for the update!  When I try out your patch, I am getting:

```
CMake Error at cmake/modules/LLVMProcessSources.cmake:113 (message):
  Found erroneous configuration for source file RISCVCounters.cpp

  LLVM's build system enforces that all source files are added to a build
  target, that exactly one build target exists in each directory, and that
  this target lists all files in that directory.  If you want multiple
  targets in the same directory, add PARTIAL_SOURCES_INTENDED to the target
  specification, though it is discouraged.
```

I'm guessing that `llvm/tools/llvm-exegesis/lib/RISCV/CMakeLists.txt` should somehow mention `RISCVCounters.cpp`.  Thoughts?


https://github.com/llvm/llvm-project/pull/120467