jit: c++-ify gdb_block

Add a constructor to gdb_block, change the name field to be a
gdb::unique_xmalloc_ptr.  This is in preparation for using an
std::forward_list<gdb_block> in the next patch.

gdb/ChangeLog:

	* jit.c (struct gdb_block): Add constructor, initialize
	real_block and next fields.
	<name>: Change type to gdb::unique_xmalloc_ptr.
	(struct gdb_symtab) <~gdb_symtab>: Free blocks with delete.
	(jit_block_open_impl): Allocate gdb_block with new.
	(finalize_symtab): Adjust to gdb::unique_xmalloc_ptr.