[llvm] fix zstd_shared detection on mingw (PR #139945)

[Petr Hosek via llvm-commits]

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@@ -29,11 +29,11 @@ find_package_handle_standard_args(
 )
 
 if(zstd_FOUND)
-  if(zstd_LIBRARY MATCHES "${zstd_STATIC_LIBRARY_SUFFIX}$")
+  if(zstd_LIBRARY MATCHES "${zstd_STATIC_LIBRARY_SUFFIX}$" AND NOT zstd_LIBRARY MATCHES "\\.dll\\.a$")
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petrhosek wrote:

I agree with @mstorsjo that it'd be preferable to use a standard variable rather than hardcoding `.dll.a` but if there isn't one then this is fine.

https://github.com/llvm/llvm-project/pull/139945