syscall.c: refactor getregs_old fallback in get_regs

syscall.c: refactor getregs_old fallback in get_regs

This change moves remaining arch specific getregs_old code into
appropriate arch subdirectories and removes unnecessary code
duplication.

* linux/getregs_old.h: New file.
* linux/powerpc/getregs_old.h: Likewise.
* linux/powerpc64/getregs_old.h: Likewise.
* linux/x86_64/getregs_old.h: Likewise.
* Makefile.am (EXTRA_DIST): Add them.
* syscall.c: Include "getregs_old.h".
[X86_64 || POWERPC]: Remove.
[ARCH_REGS_FOR_GETREGSET] (ptrace_getregset_or_getregs): Define
to ptrace_getregset.
[ARCH_REGS_FOR_GETREGS] (ptrace_getregset_or_getregs): Define
to ptrace_getregs.
(get_regs): Check for ptrace_getregset_or_getregs instead
of ARCH_REGS_FOR_GETREGSET and ARCH_REGS_FOR_GETREGS.  Use
ptrace_getregset_or_getregs instead of ptrace_getregset and
ptrace_getregs.  Check for HAVE_GETREGS_OLD instead of X86_64
and POWERPC.  Use use_getregs_old instead of getregset_support
and old_kernel.