Call disassemble_free_target in gdb

Call disassemble_free_target in gdb

Commit 20135676fc4c3912297c313b3e0d3cbd6cc402e3 ("PR24960, Memory leak
from disassembler") added "disassemble_free_target" to opcodes.  This
is used to free target-specific data when finished with a
disassembler.

This patch changes gdb to call this function where needed.

gdb/ChangeLog
2020-01-19  Tom Tromey  <tom@tromey.com>

	* disasm.c (~gdb_disassembler): New destructor.
	(gdb_buffered_insn_length): Call disassemble_free_target.
	* disasm.h (class gdb_disassembler): Declare destructor.  Use
	DISABLE_COPY_AND_ASSIGN.

Change-Id: I245ba5b7dec5e5d9f29cd21832c6e2b4fecef047