FORTIFY_SOURCE: add snprintf, vsnprintf

Add _FORTIFY_SOURCE support for snprintf, vsnprintf

At this time, we opt out of these protections for clang, as clang
does not implement __builtin_va_arg_pack().

http://clang.llvm.org/docs/UsersManual.html#c_unimpl_gcc

Change-Id: I73ebe5ec8dad1dca8898a76d6afb693a25f75375

libc/Android.mk

@@ -140,6 +140,8 @@ libc_common_src_files := \
 	stdio/vsscanf.c \
 	stdio/wbuf.c \
 	stdio/wsetup.c \
+	stdio/__snprintf_chk.c \
+	stdio/__vsnprintf_chk.c \
 	stdlib/_rand48.c \
 	stdlib/assert.c \
 	stdlib/atexit.c \