uapi: Synchronize integer register mapping

* Adopt struct user_regs_struct as a common data structure for
  user-mode register state in core dumps, ptrace, and sigcontext,
  with regard to promoting consistency in the userspace ABI.
* Disintegrate <asm/elf.h>.  Export ELF register definitions to
  userspace to satisfy <uapi/linux/elfcore.h> typedefs.
* Fix ELF_DATA definition to respect endianness.