s390: gdbarch_tdep add hook for syscall record

Most parts of s390_process_record are common for the architecture.  Only
the system call handling differs between the OSes.  In order to be able to
move s390_process_record to a common code file add a hook to record
syscalls to gdbarch_tdep.  So every OS can implement their own handling.

gdb/ChangeLog:

	* s390-linux-tdep.c (gdbarch_tdep.s390_syscall_record): New hook.
	(s390_process_record, s390_gdbarch_tdep_alloc)
	(s390_linux_init_abi_any): Use/set new hook.