Since the benchmark subproject doesn't depend on llvm,
there's not reason to generate its targets by default, i.e., there are
no dependencies and changes to llvm can't break it.
This change sets the default value for LLVM_INCLUDE_BENCHMARKS =
LLVM_BUILD_BENCHMARKS so that the targets are only generated by
default when they are actually being built.
Users can still include the targets by passing
LLVM_INCLUDE_BENCHMARKS=ON from the command line.