[llvm] [test] Make `xxhash` benchmark target name less generic (PR #100172)

[Jay Foad via llvm-commits]

jayfoad wrote:

Thanks. This changes the name of the ninja target to `bench-xxhash` and also changes the name of the binary to `benchmarks/bench-xxhash`. I am not much of a cmake expert but I am wondering:

1. Could this be a generic change that prepends `benchmarks-` to all benchmark target names, so that `ninja benchmarks-xxhash` would build `benchmarks/xxhash`, without changing the name of the executables?
2. Why do we need a named target to build benchmark executables in the first place? If I want to build it, instead of `ninja benchmarks-xxhash` I could just type `ninja benchmarks/xxhash`.

https://github.com/llvm/llvm-project/pull/100172