Unbreak Intel Apple Buck builds (#5431)

Summary:
Pull Request resolved: #5431

The code is gated on `#ifdef __aarch64__`, but the `-march` flag wasn't.

Reviewed By: cccclai

Differential Revision: D62884370

fbshipit-source-id: 944c801dd1f140a652e4cd06c7fe12ab1b85ef64

kernels/optimized/lib_defs.bzl

@@ -156,8 +156,11 @@ def define_libs():
                     ],
                 ),
             ],
-            fbobjc_compiler_flags = [
-                "-march=armv8+bf16",
+            fbobjc_platform_compiler_flags = [
+                (
+                    ".*arm64.*",
+                    ["-march=armv8+bf16"],
+                ),
             ],
             fbobjc_exported_preprocessor_flags = [
                 "-DET_BUILD_WITH_BLAS",