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

## Purpose
This patch ensures that the BLAKE3 implementation in the LLVM Support
library exports its public interface with `__declspec(dllexport)` when
building LLVM as a Windows DLL.

## Background
The effort to support building LLVM as a Windows DLL is tracked in
#109483. Additional context is provided in [this
discourse](https://discourse.llvm.org/t/psa-annotating-llvm-public-interface/85307).

## Overview
Replicate [this
logic](https://github.com/llvm/llvm-project/blob/main/llvm/cmake/modules/AddLLVM.cmake#L662-L664)
from `llvm_add_library()` for the `LLVMSupportBlake3` target. Without
this change, the `llvm_blake_` functions will only be annotated with
`__declspec(dllimport)` when building LLVM as a Windows DLL which leads
to inconsistent DLL linkage warnings from MSVC and `clang-cl`.
1 file changed
tree: f0862e20e8ff2459edcd6b6822f4a36884bd4461
  1. .ci/
  2. .github/
  3. bolt/
  4. clang/
  5. clang-tools-extra/
  6. cmake/
  7. compiler-rt/
  8. cross-project-tests/
  9. flang/
  10. flang-rt/
  11. libc/
  12. libclc/
  13. libcxx/
  14. libcxxabi/
  15. libunwind/
  16. lld/
  17. lldb/
  18. llvm/
  19. llvm-libgcc/
  20. mlir/
  21. offload/
  22. openmp/
  23. polly/
  24. pstl/
  25. runtimes/
  26. third-party/
  27. utils/
  28. .clang-format
  29. .clang-format-ignore
  30. .clang-tidy
  31. .git-blame-ignore-revs
  32. .gitattributes
  33. .gitignore
  34. .mailmap
  35. CODE_OF_CONDUCT.md
  36. CONTRIBUTING.md
  37. LICENSE.TXT
  38. pyproject.toml
  39. README.md
  40. SECURITY.md
README.md

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