arm: upgrade to _FORTIFY_SOURCE=2

Enable _FORTIFY_SOURCE=2 for ARM.

Currently, this change is a no-op, as Android's libc doesn't
support _FORTIFY_SOURCE > 1, yet.

Change-Id: Ib4c8e44e6949a37352c153e3c52a4fa001f18738
diff --git a/core/combo/TARGET_linux-arm.mk b/core/combo/TARGET_linux-arm.mk
index ca14651..deceea3 100644
--- a/core/combo/TARGET_linux-arm.mk
+++ b/core/combo/TARGET_linux-arm.mk
@@ -105,7 +105,7 @@
 			-fstack-protector \
 			-Wa,--noexecstack \
 			-Werror=format-security \
-			-D_FORTIFY_SOURCE=1 \
+			-D_FORTIFY_SOURCE=2 \
 			-fno-short-enums \
 			$(arch_variant_cflags)