handle darwin_x86_64 target cpu

--
MOS_MIGRATED_REVID=106605250
diff --git a/src/BUILD b/src/BUILD
index 00c2cb3..441bf60 100644
--- a/src/BUILD
+++ b/src/BUILD
@@ -7,6 +7,7 @@
     name = "libunix",
     srcs = select({
         ":darwin": ["//src/main/native:libunix.dylib"],
+        ":darwin_x86_64": ["//src/main/native:libunix.dylib"],
         "//conditions:default": ["//src/main/native:libunix.so"],
     }),
     visibility = [
@@ -29,6 +30,7 @@
     outs = ["install_base_key" + suffix],
     cmd = select({
         ":darwin": md5_cmd % "/sbin/md5",
+        ":darwin_x86_64": md5_cmd % "/sbin/md5",
         "//conditions:default": md5_cmd % "md5sum",
     }),
 ) for suffix, embedded_tools in {
@@ -132,6 +134,12 @@
 )
 
 config_setting(
+    name = "darwin_x86_64",
+    values = {"cpu": "darwin_x86_64"},
+    visibility = ["//visibility:public"],
+)
+
+config_setting(
     name = "freebsd",
     values = {"cpu": "freebsd"},
     visibility = ["//visibility:public"],