gdb/linux-record: Fix readdir and getdents handling

gdb/linux-record: Fix readdir and getdents handling

getdents buffer size is given in bytes, not dirent entries (which have
variable size anyway).  We don't need size_dirent and size_dirent64 for
this reason.

readdir, on the other hand, needs size of old_linux_dirent, which is
a somewhat different structure.  Accordingly, rename size_dirent
to size_old_dirent.

gdb/ChangeLog:

	* aarch64-linux-tdep.c (aarch64_linux_init_abi): Remove
	size_dirent{,64}, add size_old_dirent.
	* amd64-linux-tdep.c (amd64_linux_init_abi): Remove size_dirent{,64},
	add size_old_dirent.
	(amd64_x32_linux_init_abi): Remove size_dirent{,64}, add
	size_old_dirent.
	* arm-linux-tdep.c (arm_linux_init_abi): Remove size_dirent{,64},
	add size_old_dirent.
	* i386-linux-tdep.c (i386_linux_init_abi): Remove size_dirent{,64},
	add size_old_dirent.
	* linux-record.c (record_linux_system_call): Fix handling of readdir
	and getdents{,64}.
	* linux-record.h (struct linux_record_tdep): Remove size_dirent{,64},
	add size_old_dirent.
	* ppc-linux-tdep.c (ppc_init_linux_record_tdep): Remove
	size_dirent{,64}, add size_old_dirent.