port: only override __PCLMUL__ on MSVC

[Rexicon226]

Hello!

We ran into issues when compiling RocksDB to x86-64-v3 and other targets which have avx but not pclmul. I believe the code is doing something different from what the comment is saying. This overrides __PCLMUL__ for all compilers, not just MSVC, which caused us to bring in intrinsics that weren't supported by the target.

Is there a better way of solving this?

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