nrf/Makefile: Don't use -fno-builtin for Cortex-M0 builds.

So that error string compression is optimised correctly (it needs strcmp to
be optimised away by the compiler).

Author: dpgeorge

ports/nrf/Makefile

@@ -99,7 +99,7 @@ CFLAGS_MCU_m33 = $(CFLAGS_CORTEX_M) -mcpu=cortex-m33 -march=armv8-m.main+dsp -mc
 
 CFLAGS_MCU_m4 = $(CFLAGS_CORTEX_M) -mtune=cortex-m4 -mcpu=cortex-m4 -mfpu=fpv4-sp-d16 -mfloat-abi=hard
 
-CFLAGS_MCU_m0 = $(CFLAGS_CORTEX_M) -fshort-enums -mtune=cortex-m0 -mcpu=cortex-m0 -mfloat-abi=soft -fno-builtin
+CFLAGS_MCU_m0 = $(CFLAGS_CORTEX_M) -fshort-enums -mtune=cortex-m0 -mcpu=cortex-m0 -mfloat-abi=soft
 
 LTO ?= 1
 ifeq ($(LTO),1)