Improve platform and CPU detection, use the correct protoc binary for the running system.

Tested on OS X 10.10, Debian 8.1 (x86_64) and Debian 8.1 (i686).

--
MOS_MIGRATED_REVID=95819163
diff --git a/tools/cpp/CROSSTOOL b/tools/cpp/CROSSTOOL
index b2ed04e..cf39954 100644
--- a/tools/cpp/CROSSTOOL
+++ b/tools/cpp/CROSSTOOL
@@ -1,11 +1,16 @@
 major_version: "local"
 minor_version: ""
 default_target_cpu: "same_as_host"
+
 default_toolchain {
   cpu: "k8"
   toolchain_identifier: "local_linux"
 }
 default_toolchain {
+  cpu: "piii"
+  toolchain_identifier: "local_linux"
+}
+default_toolchain {
   cpu: "darwin"
   toolchain_identifier: "local_darwin"
 }