Several related changes:

1. Change several methods in the MachineCodeEmitter class to be pure virtual.
2. Suck emitConstantPool/initJumpTableInfo into startFunction, removing them
   from the MachineCodeEmitter interface, and reducing the amount of target-
   specific code.
3. Change the JITEmitter so that it allocates constantpools and jump tables
   *right* next to the functions that they belong to, instead of in a separate
   pool of memory.  This makes all memory for a function be contiguous, and
   means the JITEmitter only tracks one block of memory now.

llvm-svn: 28065