[cmake] Extend zstd.dll finding logic from MSVC to WIN32 in general

Extend the special logic for finding `zstd.dll` in `Findzstd` to apply
to all Windows configurations rather than just MSVC.  From what
I understand, this naming scheme is specific to Windows in general,
rather than "Windows with MSVC", and `.lib` files are always used to
link to shared libraries.  I could reproduce the original problem when
using Clang with Conda-installed LLVM, and extending the logic to
`WIN32` seems to fix it for me.

That said, I'm not an expert on Windows and I have only done very
limited testing, so I'd appreciate if someone could double check that
I'm not breaking some workflow.

Fixes llvm#121345

llvm/cmake/modules/Findzstd.cmake

@@ -10,7 +10,7 @@
 # zstd::libzstd_shared
 # zstd::libzstd_static
 
-if(MSVC)
+if(WIN32)
   set(zstd_STATIC_LIBRARY_SUFFIX "_static\\${CMAKE_STATIC_LIBRARY_SUFFIX}$")
 else()
   set(zstd_STATIC_LIBRARY_SUFFIX "\\${CMAKE_STATIC_LIBRARY_SUFFIX}$")
@@ -33,7 +33,7 @@ if(zstd_FOUND)
     set(zstd_STATIC_LIBRARY "${zstd_LIBRARY}")
   elseif (NOT TARGET zstd::libzstd_shared)
     add_library(zstd::libzstd_shared SHARED IMPORTED)
-    if(MSVC)
+    if(WIN32)
       include(GNUInstallDirs) # For CMAKE_INSTALL_LIBDIR and friends.
       # IMPORTED_LOCATION is the path to the DLL and IMPORTED_IMPLIB is the "library".
       get_filename_component(zstd_DIRNAME "${zstd_LIBRARY}" DIRECTORY)