libs/libxx: Enforce RTTI for building libcxxabi

It is okay for the application to be built without RTTI (-fno-rrti), but
libcxxabi requires RTTI at least for the library.

Signed-off-by: Gustavo Henrique Nihei <gustavo.nihei@espressif.com>

libs/libxx/libcxxabi.defs

@@ -55,6 +55,9 @@ else
 CPPSRCS += cxa_noexception.cpp
 endif
 
+# RTTI is required for building the libcxxabi library
+CXXFLAGS += -frtti
+
 DEPPATH += --dep-path libcxxabi/src
 VPATH += libcxxabi/src