Add partial support for g++ code compiled with -fvtable-thunks.

	Add partial support for g++ code compiled with -fvtable-thunks.
	* c-valprint.c (c_val_print):  Add vtblprint support
	when using thunks.
	* cp-valprint.c (cp_is_vtbl_member):  A vtable can be an array of
	pointers (if using thunks) as well as array of structs (otherwise).
	* cp-valprint.c (vtbl_ptr_name_old, vtbl_ptr_name):  Move to global
	level, and make the latter non-static (so define_symbol can use it).
	* stabsread.c (define_symbol):  If the type being defined is a
	pointer type named "__vtbl_ptr_type", set the TYPE_NAME to that name.
	* symtab.h (VTBL_PREFIX_P):  Allow "_VT" as well as "_vt".
	* values.c (value_virtual_fn_field):  Handle thunks.
	* values.c (value_headof):  Minor efficiency hack.
	* values.c (value_headof):   Incomplete thunk support.  FIXME.