s390, s390x: use PTRACE_GETREGSET to fetch registers

s390, s390x: use PTRACE_GETREGSET to fetch registers

* linux/s390/arch_regs.h: New file.
* linux/s390x/arch_regs.h: New file.
* Makefile.am (EXTRA_DIST): Add them.
* signal.c (sys_sigreturn) [S390 || S390X]: Use s390_frame_ptr.
* syscall.c [S390 || S390X] (s390_regset, s390_frame_ptr): New variable.
[S390 || S390X] (ARCH_REGS_FOR_GETREGSET): New macro.
(print_pc) [S390 || S390X]: Use s390_regset.
(get_scno) [S390 || S390X]: Likewise.
(get_syscall_args) [S390 || S390X]: Likewise.
(get_error) [S390 || S390X]: Likewise.
(get_syscall_result) [S390 || S390X]: Remove.