libcc1: share the GCC interface code

Both the C and C++ side of the GDB plugin in libcc1 share a lot of
code relating to the base GCC interface.  It was all copy-and-pasted,
but is essentially identical between the two.  This is by design, as
the base GCC API is intended to be shared.

This patch merges the implementations into base_gdb_plugin, which was
introduced earlier for this purpose.

libcc1

	* libcp1.cc (libcp1): Change parameters.  Update.
	(libcp1_set_verbose, libcp1_set_arguments)
	(libcp1_set_triplet_regexp, libcp1_set_driver_filename)
	(libcp1_set_source_file, libcp1_set_print_callback, fork_exec)
	(libcp1_compile, libcp1_destroy, vtable): Remove.
	(libcp1::add_callbacks): New method, extracted from
	libcp1_compile.
	(gcc_c_fe_context): Update.
	* libcc1.cc (libcc1): Change parameters.  Update.
	(libcc1_set_verbose, libcc1_set_arguments)
	(libcc1_set_triplet_regexp, libcc1_set_driver_filename)
	(libcc1_set_source_file, libcc1_set_print_callback, fork_exec)
	(libcc1_compile, libcc1_destroy, vtable): Remove.
	(libcc1::add_callbacks): New method, extracted from
	libcc1_compile.
	(gcc_c_fe_context): Update.
	* gdbctx.hh (base_gdb_plugin): Change parameters.
	(~base_gdb_plugin): New.
	<add_callbacks>: New virtual method.
	<plugin_name, fe_version, compiler_name, vtable>: New members.
	(get_self, do_set_verbose, do_set_arguments)
	(do_set_triplet_regexp, do_set_driver_filename)
	(do_set_arguments_v0, do_set_source_file, do_set_print_callback)
	(fork_exec, do_compile, do_compile_v0, do_destroy): New methods.