Add pw_host_tool GN template

This change cerates a GN template called pw_host_tool which copies a
binary target to a common host_tools directory that can be added to a
user's PATH.

Change-Id: I9eb38ab4ff9d11a4a87e6be2547d5224370e3412
diff --git a/pw_vars_default.gni b/pw_vars_default.gni
index 9572a5d..c5458da 100644
--- a/pw_vars_default.gni
+++ b/pw_vars_default.gni
@@ -20,7 +20,7 @@
 # importing this file, then changing the variables as desired.
 
 # Whether to build host-side tooling.
-pw_build_host_tools = true
+pw_build_host_tools = false
 
 # Options which configure the executable targets created in Pigweed builds.
 pw_executable_config = {