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.

test/libc/runtime/test.mk

@@ -77,7 +77,7 @@ o/$(MODE)/test/libc/runtime/getenv_test.com.runs:		\
 	@HELLO=THERE build/runit $@ $<
 
 o/$(MODE)/test/libc/runtime/itsatrap_test.o: private		\
-		OVERRIDE_CFLAGS +=				\
+		CFLAGS +=					\
 			-fno-sanitize=all			\
 			-ftrapv