Create work-around wrappers for pure attribute on go_binary and go_test

This enables cgo when cross-compiling certain tests and binaries to
Linux, while disabling cgo for Windows and Darwin.

cmd/linkcheck/BUILD

@@ -1,10 +1,10 @@
 package(default_visibility = ["//visibility:public"])
 
 load(
-    "@io_bazel_rules_go//go:def.bzl",
-    "go_binary",
-    "go_library",
+    "//build:go.bzl",
+    go_binary = "go_binary_conditional_pure",
 )
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
 
 go_binary(
     name = "linkcheck",