gdb: make gdbarch_displaced_step_copy_insn return an std::unique_ptr

gdb: make gdbarch_displaced_step_copy_insn return an std::unique_ptr

This callback dynamically allocates a specialized displaced_step_closure, and
gives the ownership of the object to its caller.  So I think it would make
sense for the callback to return an std::unique_ptr, this is what this patch
implements.

gdb/ChangeLog:

	* gdbarch.sh (displaced_step_copy_insn): Change return type to an
	std::unique_ptr.
	* gdbarch.c: Re-generate.
	* gdbarch.h: Re-generate.
	* infrun.c (displaced_step_prepare_throw): Adjust to std::unique_ptr
	change.
	* aarch64-tdep.c (aarch64_displaced_step_copy_insn): Change return
	type to std::unique_ptr.
	* aarch64-tdep.h (aarch64_displaced_step_copy_insn): Likewise.
	* amd64-tdep.c (amd64_displaced_step_copy_insn): Likewise.
	* amd64-tdep.h (amd64_displaced_step_copy_insn): Likewise.
	* arm-linux-tdep.c (arm_linux_displaced_step_copy_insn): Likewise.
	* i386-linux-tdep.c (i386_linux_displaced_step_copy_insn): Likewise.
	* i386-tdep.c (i386_displaced_step_copy_insn): Likewise.
	* i386-tdep.h (i386_displaced_step_copy_insn): Likewise.
	* rs6000-tdep.c (ppc_displaced_step_copy_insn): Likewise.
	* s390-tdep.c (s390_displaced_step_copy_insn): Likewise.