build(mingw): 添加对mingw平台的支持

添加mingw平台的工具链配置，使其能够在该平台下进行构建

xmake.lua

@@ -63,6 +63,8 @@ end
 if target_plat == "switch" then
     includes("xmake/toolchains/switch.lua")
     set_toolchains("switch")
+elseif target_plat == "mingw" then
+    set_toolchains("mingw")
 end
 
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