build: Remove build system support for gold plugin, except under ChromeOS.

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 0bf51f949defa4acaf199548e94807d30b785d8d..2e817aedc4ed47094d08771e95e7d690468b8163 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -481,6 +481,8 @@ config("compiler") {
   # TODO(pcc): Make this conditional on is_official_build rather than on gn
   # flags for specific features.
   if (!is_debug && (allow_posix_link_time_opt || is_cfi) && !is_nacl) {
+    assert(use_lld || is_chromeos, "gold plugin only supported with ChromeOS")
+
     if (use_thin_lto) {
       cflags += [ "-flto=thin" ]
       ldflags += [ "-flto=thin" ]