openrisc: header file cleanups

elf.h: We can export some of these symbols to userspace.  libc needs them
and we just as well provide them as asm/elf.h as copying them into separate
libc headers.

ptrace.h: Having padding in the user_regs_struct isn't of any particular
value and just confuses GDB.  spr_defs isn't needed in userspace; libc
has its own copy anyway.

Signed-off-by: Jonas Bonn <jonas@southpole.se>