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 e28e48d4aec90171032798060c398b80f8eab23f..7995ca60607c37b32602682217240bbf12dc8b8c 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -470,6 +470,8 @@ 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, "POSIX LTO requires LLD")
+
     if (use_thin_lto) {
       cflags += [ "-flto=thin" ]
       ldflags += [ "-flto=thin" ]
@@ -477,26 +479,18 @@ config("compiler") {
       # Limit the parallelism to avoid too agressive competition between
       # linker jobs. This is still suboptimal to a potential dynamic
       # resource allocation scheme, but should be good enough.
-      if (use_lld) {
-        ldflags += [
-          "-Wl,--thinlto-jobs=8",
-          "-Wl,--thinlto-cache-dir=" +
-              rebase_path("$root_out_dir/thinlto-cache", root_build_dir),
-        ]
-      } else {
-        ldflags += [ "-Wl,-plugin-opt,jobs=8" ]
-      }
+      ldflags += [
+        "-Wl,--thinlto-jobs=8",
+        "-Wl,--thinlto-cache-dir=" +
+            rebase_path("$root_out_dir/thinlto-cache", root_build_dir),
+      ]
     } else {
       cflags += [ "-flto" ]
       ldflags += [ "-flto" ]
 
       # Apply a lower LTO optimization level as the default is too slow.
       if (is_linux) {
-        if (use_lld) {
-          ldflags += [ "-Wl,--lto-O1" ]
-        } else {
-          ldflags += [ "-Wl,-plugin-opt,O1" ]
-        }
+        ldflags += [ "-Wl,--lto-O1" ]
       } else if (is_mac) {
         ldflags += [ "-Wl,-mllvm,-O1" ]
       }
@@ -515,17 +509,10 @@ config("compiler") {
     # that is larger than the maximum call displacement, preventing the linker
     # from relocating calls (http://llvm.org/PR22999).
     if (is_linux) {
-      if (use_lld) {
-        ldflags += [
-          "-Wl,-mllvm,-function-sections",
-          "-Wl,-mllvm,-data-sections",
-        ]
-      } else {
-        ldflags += [
-          "-Wl,-plugin-opt,-function-sections",
-          "-Wl,-plugin-opt,-data-sections",
-        ]
-      }
+      ldflags += [
+        "-Wl,-mllvm,-function-sections",
+        "-Wl,-mllvm,-data-sections",
+      ]
     }
   }