GN: Factor "stamp" and "copy" details out of toolchains

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 1afc7ddf239dfc7113e90eeea9a1461736352ff2..3960acb1f342de9437a7cb4fedfa688ddf49f996 100644
--- a/build/toolchain/gcc_toolchain.gni
+++ b/build/toolchain/gcc_toolchain.gni
@@ -361,14 +361,16 @@ template("gcc_toolchain") {
       }
     }
 
+    # These two are really entirely generic, but have to be repeated in
+    # each toolchain because GN doesn't allow a template to be used here.
+    # See //build/toolchain/toolchain.gni for details.
     tool("stamp") {
-      command = "touch {{output}}"
-      description = "STAMP {{output}}"
+      command = stamp_command
+      description = stamp_description
     }
-
     tool("copy") {
-      command = "ln -f {{source}} {{output}} 2>/dev/null || (rm -rf {{output}} && cp -af {{source}} {{output}})"
-      description = "COPY {{source}} {{output}}"
+      command = copy_command
+      description = copy_description
     }
 
     # When invoking this toolchain not as the default one, these args will be