Turn on -Werror=implicit-function-declaration for arm64.

Implicit function declarations are much more dangerous on LP64 because
sizeof(int) != sizeof(void*), so any function that returns a pointer will
lose its top bits, leading to relatively hard to debug crashes.

Change-Id: Ia05beffb949ca747833c2e12c40daf896f7a60a5

core/combo/TARGET_linux-arm64.mk

@@ -91,6 +91,7 @@ TARGET_GLOBAL_CFLAGS += \
 TARGET_GLOBAL_CFLAGS += \
     -Werror=pointer-to-int-cast \
     -Werror=int-to-pointer-cast \
+    -Werror=implicit-function-declaration \
 
 TARGET_GLOBAL_CFLAGS += -fno-strict-volatile-bitfields