[llvm] [llvm] build Blake3 source with LLVM_EXPORTS defined (PR #144753)

[Andrew Rogers via llvm-commits]

andrurogerz wrote:

@compnerd, @vgvassilev this one should be straight-forward, but I thought it was worth making into its own PR for discussion if needed. Thanks!

https://github.com/llvm/llvm-project/pull/144753