Introduce a Registers class to encapsulate access to tracee registers.
Add architecture-neutral register accessors to Registers.

This assumes every architecture has an instruction pointer, a stack
pointer, a register for holding the system call number on entry to
a syscall, a (possibly different) register for the system call
result, a pseudo-register recording the original system call number
during a ptrace system call notification, and registers for six
syscall parameters.
Make util.cc use the new Register accessors.
Make replay_syscall.cc use the new Register accessors.
Make task.cc use architecture-neutral accessors on Registers.
Make replayer.cc use architecture-neutral accessors on Registers.
Make record_syscall.cc use architecture-neutral accessors to Registers.
Fix variable name in utils.h
Make recorder.cc use architecture-neutral accessors to Registers.
Merge pull request #1166 from rocallahan/refactor

Refactor