[protobuf] Add a protoc target in all toolchains

I need to be able to refer to the host toolchain target's protoc target
from any toolchain without having to spell $host_toolchain. This is to
allow some mixing of Fuchsia & Pigweed build systems.

This just creates a target in non-host toolchains that forwards to the
host toolchain.

Change-Id: Ib28496777f0f62d013dac95d3c14901b2f009e06
diff --git a/BUILD.gn b/BUILD.gn
index d6bc4ac..6eb61f6 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -532,4 +532,8 @@
     sources = [ "src/google/protobuf/compiler/main.cc" ]
     deps = [ ":protoc_lib" ]
   }
+} else {
+  group("protoc") {
+    deps = [ ":protoc($host_toolchain)" ]
+  }
 }
diff --git a/BUILD.input.gn b/BUILD.input.gn
index 5afc49a..9ab9aab 100644
--- a/BUILD.input.gn
+++ b/BUILD.input.gn
@@ -208,4 +208,8 @@
     sources = [ "src/google/protobuf/compiler/main.cc" ]
     deps = [ ":protoc_lib" ]
   }
+} else {
+  group("protoc") {
+    deps = [ ":protoc($host_toolchain)" ]
+  }
 }