[test] Make xxhash benchmark target name less generic

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

See llvm#99634 (comment)

Author: BertalanD

llvm/benchmarks/CMakeLists.txt

@@ -2,4 +2,4 @@ set(LLVM_LINK_COMPONENTS
   Support)
 
 add_benchmark(DummyYAML DummyYAML.cpp PARTIAL_SOURCES_INTENDED)
-add_benchmark(xxhash xxhash.cpp PARTIAL_SOURCES_INTENDED)
+add_benchmark(bench-xxhash xxhash.cpp PARTIAL_SOURCES_INTENDED)