gdb: turn gdb::bcache's function pointers into virtual methods

The two function pointers optionally passed to gdb::bcache are very good
candidates to be turned into virtual methods, this patch does that in
the most straightforward / unsurprising way.

gdb/ChangeLog:

	* bcache.h (struct bcache) <bcache>: Remove constructor.
	<m_hash_function, m_compare_function>: Remove.
	<~bcache>: Make virtual.
	<compare>: Remove static method, introduce virtual method.
	<default_hash>: Remove.
	<hash>: New virtual method.
	* bcache.c (bcache::expand_hash_table): Update.
	(bcache::insert): Update.
	(bcache::hash): New.
	(bcache::compare): Update comment and parameter names.
	* gdbtypes.c (types_deeply_equal): Update.
	* psymtab.h (struct psymbol_bcache): New struct.
	(class psymtab_storage) <psymtab_storage>: Make default.
	<psymbol_cache>: Change type to psymbol_bcache.
	* psymtab.c (psymtab_storage::psymtab_storage): Remove.
	(psymbol_hash): Change to...
	(psymbol_bcache::hash): ... this.
	(psymbol_compare): Change to...
	(psymbol_bcache::compare): ... this.

Change-Id: I41d578e61de8ac1163461a28fbd220d1f855e372