Add curve25519/asm/x25519-asm-x86_64.S.

Change-Id: I5feff96d8d80981e72a8b3aa3fd90e3202dff39e
Reviewed-on: https://boringssl-review.googlesource.com/6903
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/util/generate_build_files.py b/util/generate_build_files.py
index 6321c5c..acc693a 100644
--- a/util/generate_build_files.py
+++ b/util/generate_build_files.py
@@ -44,6 +44,9 @@
         'src/crypto/curve25519/asm/x25519-asm-arm.S',
         'src/crypto/poly1305/poly1305_arm_asm.S',
     ],
+    ('linux', 'x86_64'): [
+        'src/crypto/curve25519/asm/x25519-asm-x86_64.S',
+    ],
 }