Fix dynamic_cast across dlopen boundaries.

Enabling _LIBCXX_DYNAMIC_FALLBACK allows type_info objects to be equal
if their strings are equal, not just their addresses. This is very
common on Android where libraries are loaded with dlopen via
System.loadLibrary.

This behavior matches libsupc++, which always does a string compare to
preserve compatibility for plugin architectures that behave similarly.

Gory details are in the bug.

Test: ndk/run_tests.py --filter dynamic_cast_dlopen
Bug: android/ndk#533
Change-Id: I26cbf8d260cb7fb924580db6b346b42f39d5c2ed

Android.mk

@@ -46,6 +46,16 @@ libcxxabi_includes := \
 libcxxabi_cflags := -D__STDC_FORMAT_MACROS
 libcxxabi_cppflags := -std=c++11 -Wno-unknown-attributes
 
+# Enabling _LIBCXX_DYNAMIC_FALLBACK allows type_info objects to be equal if
+# their strings are equal, not just their addresses. This is very common on
+# Android where libraries are loaded as RTLD_LOCAL via System.loadLibrary.
+#
+# This behavior matches libsupc++, which always does a string compare to
+# preserve compatibility for plugin architectures that behave similarly.
+#
+# https://github.com/android-ndk/ndk/issues/533
+libcxxabi_cppflags += -D_LIBCXX_DYNAMIC_FALLBACK
+
 ifneq (,$(filter armeabi%,$(TARGET_ARCH_ABI)))
     use_llvm_unwinder := true
     libcxxabi_cppflags += -DLIBCXXABI_USE_LLVM_UNWINDER=1