Add pub-sub.{h,cc}

Add pub-sub.{h,cc}

This patch introduces a publish/subscribe mechanism, allowing for
loosely-coupled senders and receivers, with strongly-typed messages
passing between them.  For example, a GCC subsystem could publish
messages about events, and a plugin could subscribe to them.

An example can be seen in the selftests.

gcc/ChangeLog:
	* Makefile.in (OBJS-libcommon): Add pub-sub.o.
	* pub-sub.cc: New file.
	* pub-sub.h: New file.
	* selftest-run-tests.cc (selftest::run_tests): Call
	selftest::pub_sub_cc_tests.
	* selftest.h (selftest::pub_sub_cc_tests): New decl.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>