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.

libc/fmt/fmt.mk

@@ -55,14 +55,14 @@ $(LIBC_FMT_A).pkg:				\
 		$(foreach x,$(LIBC_FMT_A_DIRECTDEPS),$($(x)_A).pkg)
 
 $(LIBC_FMT_A_OBJS): private			\
-		OVERRIDE_CFLAGS +=		\
+		CFLAGS +=			\
 			-fno-jump-tables
 
 o/$(MODE)/libc/fmt/formatint64.o		\
 o/$(MODE)/libc/fmt/formatint64thousands.o	\
 o/$(MODE)/libc/fmt/dosdatetimetounix.o		\
 o/$(MODE)/libc/fmt/itoa64radix10.greg.o: private\
-		OVERRIDE_CFLAGS +=		\
+		CFLAGS +=			\
 			-O3
 
 o/$(MODE)/libc/fmt/atoi.o			\
@@ -74,7 +74,7 @@ o/$(MODE)/libc/fmt/strtoimax.o			\
 o/$(MODE)/libc/fmt/strtoumax.o			\
 o/$(MODE)/libc/fmt/wcstoimax.o			\
 o/$(MODE)/libc/fmt/wcstoumax.o: private		\
-		OVERRIDE_CFLAGS +=		\
+		CFLAGS +=			\
 			-Os
 
 # we can't use compiler magic because:
@@ -82,7 +82,7 @@ o/$(MODE)/libc/fmt/wcstoumax.o: private		\
 o/$(MODE)/libc/fmt/strerrno.greg.o		\
 o/$(MODE)/libc/fmt/strerrdoc.greg.o		\
 o/$(MODE)/libc/fmt/strerror_wr.greg.o: private	\
-		OVERRIDE_CFLAGS +=		\
+		CFLAGS +=			\
 			-fpie			\
 			-ffreestanding		\
 			$(NO_MAGIC)