Exclude xa_nn_activations_asym8_asym8.c from NNLib build for Hifi4 (#2384)

Due to an assembler error:
/tmp/xa_nn_activations_asym8_asym8-037a85.s: Assembler messages: /tmp/xa_nn_activations_asym8_asym8-037a85.s:470: Error: operand 3 of 'slli' has invalid value '0' clang-3.9: error: Xtensa-as command failed with exit code 1 (use -v to see invocation)
BUG=319139235

tensorflow/lite/micro/tools/make/ext_libs/xtensa.inc

diff --git a/tensorflow/lite/micro/tools/make/ext_libs/xtensa.inc b/tensorflow/lite/micro/tools/make/ext_libs/xtensa.inc
index 2d49744..b6db417 100644
--- a/tensorflow/lite/micro/tools/make/ext_libs/xtensa.inc
+++ b/tensorflow/lite/micro/tools/make/ext_libs/xtensa.inc
@@ -139,6 +139,11 @@
       $(NNLIB_PATH)/algo/ndsp/hifi4/src/vec_tanhf_hifi4.c \
       $(NNLIB_PATH)/algo/ndsp/hifi4/src/tanhf_tbl.c
   endif
+  
+  ifeq ($(TARGET_ARCH), hifi4)
+    EXCLUDED_NNLIB_SRCS += \
+      $(NNLIB_PATH)/algo/kernels/activations/hifi4/xa_nn_activations_asym8_asym8.c
+  endif
 
   THIRD_PARTY_KERNEL_CC_SRCS := $(filter-out $(EXCLUDED_NNLIB_SRCS), $(THIRD_PARTY_KERNEL_CC_SRCS))