revert: "ci: remove mingw job #18580"

This partially reverts commit f8af81445b.

The mingw parts of cmake was removed to see if it was still used
(ref: https://github.com/neovim/neovim/pull/18580). It turns out it is,
so this will fix that.

Closes: https://github.com/neovim/neovim/issues/18597

third-party/cmake/BuildLuv.cmake

@@ -91,7 +91,16 @@ if(USE_BUNDLED_LIBUV)
     -DCMAKE_PREFIX_PATH=${DEPS_INSTALL_DIR})
 endif()
 
-if(MSVC)
+if(MINGW AND CMAKE_CROSSCOMPILING)
+  get_filename_component(TOOLCHAIN ${CMAKE_TOOLCHAIN_FILE} REALPATH)
+  set(LUV_CONFIGURE_COMMAND
+    ${LUV_CONFIGURE_COMMAND_COMMON}
+    # Pass toolchain
+    -DCMAKE_TOOLCHAIN_FILE=${TOOLCHAIN}
+    "-DCMAKE_C_FLAGS:STRING=${LUV_INCLUDE_FLAGS} -D_WIN32_WINNT=0x0600"
+    # Hack to avoid -rdynamic in Mingw
+    -DCMAKE_SHARED_LIBRARY_LINK_C_FLAGS="")
+elseif(MSVC)
   set(LUV_CONFIGURE_COMMAND
     ${LUV_CONFIGURE_COMMAND_COMMON}
     -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}