libxx: Add CXX_STANDARD to select -std=c++??

and default to "c++17"

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

libs/libxx/Kconfig

@@ -85,6 +85,12 @@ endchoice
 
 endif
 
+config CXX_STANDARD
+	string "Language standard"
+	default "c++17"
+	---help---
+		Possible values: c++98, c++11, c++14, c++17 and c++20
+
 config CXX_EXCEPTION
 	bool "Enable Exception Support"
 

libs/libxx/libcxx.defs

@@ -50,7 +50,6 @@ ifeq ($(wildcard libcxx/.git),)
 	$(call DELDIR, libcxx)
 endif
 
-CXXFLAGS += -std=c++17
 CXXFLAGS += ${DEFINE_PREFIX}_LIBCPP_BUILDING_LIBRARY
 
 ifeq ($(CONFIG_LIBSUPCXX), y)