[test] Make xxhash benchmark target name less generic

[BertalanD]

Downstream users reported that the xxhash target name conflicts with some of their existing build targets. Rename it to bench-xxhash.

See #99634 (comment)

[hiraditya]

LGTM, not sure why the build failed. I have rebased, hopefully the test will pass.

[jayfoad]

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.