C++: prepare RTTI support

Ref. https://github.com/espressif/esp-idf/issues/1684

Also, for full RTTI support, libstdc++.a in the toolchain should be built
in both with RTTI and w/o RTTI options. Multilib with -fno-rtti
flag is used for that.

Note that this commit does not actually enable RTTI support.
The respective Kconfig option is hidden, and will be made visible when
the toolchain is updated.

make/project.mk

@@ -452,7 +452,6 @@ CXXFLAGS ?=
 EXTRA_CXXFLAGS ?=
 CXXFLAGS := $(strip \
 	-std=gnu++11 \
-	-fno-rtti \
 	$(OPTIMIZATION_FLAGS) $(DEBUG_FLAGS) \
 	$(COMMON_FLAGS) \
 	$(COMMON_WARNING_FLAGS) \
@@ -465,6 +464,13 @@ else
 CXXFLAGS += -fno-exceptions
 endif
 
+ifdef CONFIG_COMPILER_CXX_RTTI
+CXXFLAGS += -frtti
+else
+CXXFLAGS += -fno-rtti
+LDFLAGS += -fno-rtti
+endif
+
 ARFLAGS := cru
 
 export CFLAGS CPPFLAGS CXXFLAGS ARFLAGS