scons: Set -static-libstdc++ on mingw-w64

To avoid depending on libstdc++-xxx.dll

scons/gallium.py

@@ -537,6 +537,8 @@ def generate(env):
         if env['platform'] == 'windows':
             # Avoid depending on gcc runtime DLLs
             linkflags += ['-static-libgcc']
+            if env['machine'] == 'x86_64':
+                linkflags += ['-static-libstdc++']
             # Handle the @xx symbol munging of DLL exports
             shlinkflags += ['-Wl,--enable-stdcall-fixup']
             #shlinkflags += ['-Wl,--kill-at']