Refactor the machine code emitter interface to pull the pointers for the current

code emission location into the base class, instead of being in the derived classes.

This change means that low-level methods like emitByte/emitWord now are no longer
virtual (yaay for speed), and we now have a framework to support growable code
segments.  This implements feature request #1 of PR469.

llvm-svn: 28059