Remove GN forwarding of the target variables.

build/toolchain/gcc_toolchain.gni

Index: build/toolchain/gcc_toolchain.gni
diff --git a/build/toolchain/gcc_toolchain.gni b/build/toolchain/gcc_toolchain.gni
index 163643fdd5b03dbb9c33d45612c6020e2c9ce184..ad2d1339190017182da2a9db1b016922802fb48c 100644
--- a/build/toolchain/gcc_toolchain.gni
+++ b/build/toolchain/gcc_toolchain.gni
@@ -99,14 +99,11 @@ template("gcc_toolchain") {
       # Populate toolchain args from the invoker.
       forward_variables_from(invoker_toolchain_args, "*")
 
-      # These values need to be passed through unchanged to all secondary
-      # toolchains. BUILDCONFIG.gn sets some defaults based on the values of
-      # the operating system and compiler, and we want to force the values to
-      # be consistent if re-running the computation in another context leads
-      # to different defaults.
+      # The host toolchain value computed by the default toolchain's setup
+      # needs to be passed through unchanged to all secondary toolchains to
+      # ensure that it's always the same, regardless of the values that may be
+      # set on those toolchains.
       host_toolchain = host_toolchain
-      target_os = target_os
-      target_cpu = target_cpu
 
       if (!defined(invoker_toolchain_args.v8_current_cpu)) {
         v8_current_cpu = invoker_toolchain_args.current_cpu