toolchaini/gcc: fix libstdc++ dual abi model

libstdcxx-dual-abi needs to be enabled to actually support C++11 ABI.
Enable the config flag to also permit support of .NET 6 development on
OpenWrt.

Signed-off-by: Ivan Maslov <[email protected]>
[ reword commit description and title ]
Signed-off-by: Christian Marangi <[email protected]>

toolchain/gcc/common.mk

@@ -119,7 +119,7 @@ GCC_CONFIGURE:= \
 		--disable-decimal-float \
 		--with-diagnostics-color=auto-if-env \
 		--enable-__cxa_atexit \
-		--disable-libstdcxx-dual-abi \
+		--enable-libstdcxx-dual-abi \
 		--with-default-libstdcxx-abi=new
 ifneq ($(CONFIG_mips)$(CONFIG_mipsel),)
   GCC_CONFIGURE += --with-mips-plt