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

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> @dtcxzyw Can you please show the build options and flags so I can reproduce this?

```
cmake llvm-project/llvm -DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED_LIBS=ON -G Ninja \
        -DLLVM_TARGETS_TO_BUILD="RISCV -DLLVM_PARALLEL_LINK_JOBS=1 -DLLVM_ENABLE_ASSERTIONS=ON
cmake --build . -j -t llvm-exegesis
```

Compiled with gcc:
```
dtcxzyw at bananapif3:~$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/riscv64-linux-gnu/13/lto-wrapper
Target: riscv64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 13.2.0-23ubuntu4' --with-bugurl=file:///usr/share/doc/gcc-13/README.Bugs --enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-13 --program-prefix=riscv64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/libexec --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-libstdcxx-backtrace --enable-gnu-unique-object --disable-libitm --disable-libquadmath --disable-libquadmath-support --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=release --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --disable-multilib --with-arch=rv64gc --with-abi=lp64d --enable-checking=release --build=riscv64-linux-gnu --host=riscv64-linux-gnu --target=riscv64-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 13.2.0 (Ubuntu 13.2.0-23ubuntu4)
```


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