Add support for cross-compiling x86-64 binaries on M1 Mac.

Change-Id: I5e4216aef7d1dca2dfc54dc1228d0fd614e5960e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/648296
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
diff --git a/gn/skia/BUILD.gn b/gn/skia/BUILD.gn
index 47a3127..bed7053 100644
--- a/gn/skia/BUILD.gn
+++ b/gn/skia/BUILD.gn
@@ -279,6 +279,19 @@
         "-target",
         "arm64-apple-macos11",
       ]
+    } else {
+      asmflags += [
+        "-target",
+        "x86_64-apple-macos10.13",
+      ]
+      cflags += [
+        "-target",
+        "x86_64-apple-macos10.13",
+      ]
+      ldflags += [
+        "-target",
+        "x86_64-apple-macos10.13",
+      ]
     }
   }