Use select to disable building static binaries if --cpu=darwin.

This change allows kubectl to be built on MacOS machines using
`bazel build //cmd/kubectl`.

Mac OS X doesn't support static binaries because it does not have a
stable syscall API. Userspace binaries are expected to dynamically
link against libcrt instead.

https://developer.apple.com/library/content/qa/qa1118/_index.html

cmd/kubectl/BUILD

@@ -7,12 +7,17 @@ load("//pkg/version:def.bzl", "version_x_defs")
 
 go_binary(
     name = "kubectl",
-    gc_linkopts = [
+    gc_linkopts = select({
+        # Mac OS X doesn't support static binaries:
+        # https://developer.apple.com/library/content/qa/qa1118/_index.html
+        "@io_bazel_rules_go//go/platform:darwin_amd64": [],
+        "//conditions:default": [
             "-linkmode",
             "external",
             "-extldflags",
             "-static",
         ],
+    }),
     library = ":go_default_library",
     visibility = [
         "//build/visible_to:COMMON_release",