C++-ify ravenscar_arch_ops

C++-ify ravenscar_arch_ops

This turns ravenscar_arch_ops into an abstract base class and updates
all the places where it is used.  This is an improvement because it
avoids any possibility of forgetting to set one of the function
pointers.  It also makes clear that these functions aren't intended to
be changed dynamically.

This version of the patch removes the prepare_to_store method, as it
is unused, and it is easy enough to add if it is ever needed.

gdb/ChangeLog
2019-02-15  Tom Tromey  <tromey@adacore.com>

	* sparc-ravenscar-thread.c (struct sparc_ravenscar_ops): Derive
	from ravenscar_arch_ops.
	(sparc_ravenscar_ops::fetch_registers)
	(sparc_ravenscar_ops::store_registers): Now methods.
	(sparc_ravenscar_prepare_to_store): Remove.
	(sparc_ravenscar_ops): Redefine.
	* ravenscar-thread.h (struct ravenscar_arch_ops): Add virtual
	methods and destructor.  Remove members.
	* ravenscar-thread.c (ravenscar_thread_target::fetch_registers)
	(ravenscar_thread_target::store_registers)
	(ravenscar_thread_target::prepare_to_store): Update.
	* ppc-ravenscar-thread.c (ppc_ravenscar_generic_prepare_to_store):
	Remove.
	(struct ppc_ravenscar_powerpc_ops): Derive from
	ravenscar_arch_ops.
	(ppc_ravenscar_powerpc_ops::fetch_registers)
	(ppc_ravenscar_powerpc_ops::store_registers): Now methods.
	(ppc_ravenscar_powerpc_ops): Redefine.
	(struct ppc_ravenscar_e500_ops): Derive from ravenscar_arch_ops.
	(ppc_ravenscar_e500_ops::fetch_registers)
	(ppc_ravenscar_e500_ops::store_registers): Now methods.
	(ppc_ravenscar_e500_ops): Redefine.
	* aarch64-ravenscar-thread.c
	(aarch64_ravenscar_generic_prepare_to_store): Remove.
	(struct aarch64_ravenscar_ops): Derive from ravenscar_arch_ops.
	(aarch64_ravenscar_fetch_registers)
	(aarch64_ravenscar_store_registers): Now methods.
	(aarch64_ravenscar_ops): Redefine.