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

build/toolchain/toolchain.gni

Index: build/toolchain/toolchain.gni
diff --git a/build/toolchain/toolchain.gni b/build/toolchain/toolchain.gni
index 5bd7d9c714bf6928c4e373da0093fb5e2e096496..e7f43792af1b4c76ce0f7666b583ed267a60f69e 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -38,3 +38,20 @@ if (is_posix) {
 } else {
   shlib_prefix = ""
 }
+
+# While other "tool"s in a toolchain are specific to the target of that
+# toolchain, the "stamp" and "copy" tools are really generic to the host;
+# but each toolchain must define them separately.  GN doesn't allow a
+# template instantiation inside a toolchain definition, so some boilerplate
+# has to be repeated in each toolchain to define these two tools.  These
+# four variables reduce the duplication in that boilerplate.
+stamp_description = "STAMP {{output}}"
+copy_description = "COPY {{source}} {{output}}"
+if (host_os == "win") {
+  stamp_command = "$python_path gyp-win-tool stamp {{output}}"
+  copy_command =
+      "$python_path gyp-win-tool recursive-mirror {{source}} {{output}}"
+} else {
+  stamp_command = "touch {{output}}"
+  copy_command = "ln -f {{source}} {{output}} 2>/dev/null || (rm -rf {{output}} && cp -af {{source}} {{output}})"
+}