Make BLAKE3 a component library

It's unusual that BLAKE3/CMakeLists.txt just defines a list of
files that it injects into its parent scope. The list should either
be defined in llvm/lib/Support/CMakeLists.txt, or
llvm/lib/Support/BLAKE3/CMakeLists.txt should define an object
library.

This does the latter. It makes llvm/lib/Support/BLAKE3/CMakeLists.txt
more self-contained.

No behavior change.

Differential Revision: https://reviews.llvm.org/D122428