TFLu: remove -fno-builtin compiler flag

The flag may cause performance issues, since it disables special
handling and optimizations of standard C library functions.

tensorflow/lite/micro/tools/make/targets/apollo3evb_makefile.inc

@@ -40,7 +40,6 @@ $(MAKEFILE_DIR)/downloads/$(AM_SDK_DEST)/$(SF_BSPS_DEST): $(MAKEFILE_DIR)/downlo
     -fmessage-length=0 \
     -fno-exceptions \
     -fno-unwind-tables \
-    -fno-builtin \
     -ffunction-sections \
     -fdata-sections \
     -funsigned-char \

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

@@ -19,7 +19,6 @@ ifeq ($(TARGET), bluepill)
     -fmessage-length=0 \
     -fno-exceptions \
     -fno-unwind-tables \
-    -fno-builtin \
     -ffunction-sections \
     -fdata-sections \
     -funsigned-char \

tensorflow/lite/micro/tools/make/targets/ecm3531_makefile.inc

@@ -27,7 +27,6 @@ ifeq ($(TARGET), ecm3531)
     -fmessage-length=0 \
     -fno-exceptions \
     -fno-unwind-tables \
-    -fno-builtin \
     -ffunction-sections \
     -fdata-sections \
     -funsigned-char \

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

@@ -39,7 +39,6 @@ ifeq ($(TARGET), hexagon)
     -fdata-sections \
     -ffunction-sections \
     -fmessage-length=0 \
-    -fno-builtin \
     -fno-delete-null-pointer-checks \
     -fno-exceptions \
     -fno-register-global-dtors-with-atexit \

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

@@ -16,7 +16,6 @@ ifeq ($(TARGET), riscv32_mcu)
     -DTF_LITE_MCU_DEBUG_LOG \
     -DTF_LITE_USE_GLOBAL_ROUND \
     -fno-unwind-tables \
-    -fno-builtin \
     -ffunction-sections \
     -fdata-sections \
     -funsigned-char \

tensorflow/lite/micro/tools/make/targets/stm32f4_makefile.inc

@@ -16,7 +16,6 @@ ifeq ($(TARGET), stm32f4)
     -fmessage-length=0 \
     -fno-exceptions \
     -fno-unwind-tables \
-    -fno-builtin \
     -ffunction-sections \
     -fdata-sections \
     -funsigned-char \