Get --ftrace working on aarch64

This change implements a new approach to function call logging, that's
based on the GCC flag: -fpatchable-function-entry. Read the commentary
in build/config.mk to learn how it works.

third_party/mbedtls/mbedtls.mk

@@ -48,27 +48,27 @@ $(THIRD_PARTY_MBEDTLS_A).pkg:						\
 		$(foreach x,$(THIRD_PARTY_MBEDTLS_A_DIRECTDEPS),$($(x)_A).pkg)
 
 $(THIRD_PARTY_MBEDTLS_A_OBJS): private					\
-			OVERRIDE_CFLAGS +=				\
+			CFLAGS +=					\
 				-fdata-sections				\
 				-ffunction-sections
 
 o/$(MODE)/third_party/mbedtls/everest.o: private			\
-			OVERRIDE_CFLAGS +=				\
+			CFLAGS +=					\
 				-O3
 
 o/$(MODE)/third_party/mbedtls/bigmul4.o					\
 o/$(MODE)/third_party/mbedtls/bigmul6.o: private			\
-			OVERRIDE_CFLAGS +=				\
+			CFLAGS +=					\
 				-O2
 
 ifeq ($(ARCH), x86_64)
 o/$(MODE)/third_party/mbedtls/shiftright-avx.o: private			\
-			OVERRIDE_CFLAGS +=				\
+			CFLAGS +=					\
 				-O3 -mavx
 endif
 
 o/$(MODE)/third_party/mbedtls/zeroize.o: private			\
-			OVERRIDE_CFLAGS +=				\
+			CFLAGS +=					\
 				-O3					\
 				-x-no-pg				\
 				-fomit-frame-pointer			\