Add command line option to select -stdlib=libc++ with Xtensa.

tensorflow/lite/micro/tools/make/targets/xtensa_makefile.inc

@@ -9,6 +9,7 @@
 #      For example: hifimini
 
 TARGET_ARCH :=
+XTENSA_USE_LIBC :=
 
 ifndef XTENSA_BASE
   $(error XTENSA_BASE is undefined)
@@ -36,7 +37,6 @@ PLATFORM_FLAGS = \
   -DTF_LITE_USE_CTIME \
   --xtensa-core=$(XTENSA_CORE) \
   -mcoproc \
-  -stdlib=libc++ \
   -DMAX_RFFT_PWR=9 \
   -DMIN_RFFT_PWR=MAX_RFFT_PWR \
   $(TARGET_ARCH_DEFINES)
@@ -50,6 +50,21 @@ TARGET_TOOLCHAIN_PREFIX := xt-
 CXX_TOOL := clang++
 CC_TOOL := clang
 
+# Unused exception related symbols make their way into a binary that links
+# against TFLM as described in https://github.com/tensorflow/tensorflow/issues/47575.
+# We have two options to avoid this. The first involves using -stdlib=libc++ and
+# the second involves stubbing out and modifying some of the files in the Xtensa
+# toolchain to prevent inclusion of the exception handling code
+# (http://b/182209217#comment3). This Makefile supports building TFLM in a way
+# that is compatible with either of the two approaches.
+ifeq ($(XTENSA_USE_LIBC), true)
+  PLATFORM_FLAGS += -stdlib=libc++
+else
+  # TODO(b/150240249): Do not filter-out -fno-rtti once that works for the
+  # Xtensa toolchain.
+  CXXFLAGS := $(filter-out -fno-rtti, $(CXXFLAGS))
+endif
+
 CXXFLAGS += $(PLATFORM_FLAGS)
 CCFLAGS += $(PLATFORM_FLAGS)