Add ffs and memcmp16 to x86_64.

Change-Id: I652c1356f1c7c52299977181c2cf154386979380
diff --git a/libc/arch-x86_64/x86_64.mk b/libc/arch-x86_64/x86_64.mk
index 244eb56..b17b138 100644
--- a/libc/arch-x86_64/x86_64.mk
+++ b/libc/arch-x86_64/x86_64.mk
@@ -9,6 +9,8 @@
     arch-x86_64/bionic/__set_tls.c \
     arch-x86_64/bionic/sigsetjmp.S \
     arch-x86_64/bionic/syscall.S \
+    arch-x86_64/string/ffs.S \
+    string/memcmp16.c \
 
 _LIBC_ARCH_STATIC_SRC_FILES := \
     bionic/dl_iterate_phdr_static.c \