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

and default to "c++17"

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

arch/sparc/src/sparc_v8/Toolchain.defs

@@ -82,6 +82,10 @@ ARCHCXXFLAGS += -fno-common -nostdinc++
 ARCHCFLAGS += -Wall -Wstrict-prototypes -Wshadow -Wundef
 ARCHCXXFLAGS += -Wall -Wshadow -Wundef
 
+ifneq ($(CONFIG_CXX_STANDARD),)
+  ARCHCXXFLAGS += -std=$(CONFIG_CXX_STANDARD)
+endif
+
 ifneq ($(CONFIG_CXX_EXCEPTION),y)
   ARCHCXXFLAGS += -fno-exceptions -fcheck-new
 endif