Fix version negotiation in libcc1 plugins

Fix version negotiation in libcc1 plugins

This fixes version negotiation in the libcc1 plugins.  It's done in a
simple way: the version number from the context object is now passed
to base_gdb_plugin.

The idea behind this is that when the client (gdb) requests version N,
the plugin should respond with the newest version that it knows of
that is backward compatible to N.  That is, the connection can be
upgraded.  Note that the protocol does not change much, and no
backward incompatibilities have ever been needed.

The C plugin is also changed to advertise GCC_C_FE_VERSION_1.

The version negotiation approach should of course be documented, but I
did that in a subsequent patch, in order to only have one patch
touching the 'include' directory and thus needing a merge to
binutils-gdb.

libcc1

	* libcp1.cc (libcp1::libcp1): Use FE version number from context.
	* libcc1.cc (libcc1::libcc1): Use FE version number from context.
	(c_vtable): Use GCC_C_FE_VERSION_1.