[Jikesrvm-core] improvements to PowerPC version of VM_Assembler

[David P G. <gr...@us...>]

I've checked in enhancements to the PowerPC version of VM_Assembler to give
it the same support for manually managed forward references as the IA32
VM_Assembler.  Specifically, there are new functions emitForwardB,
emitForwardBC, and emitForwardBL that return a VM_ForwardReference object.
This avoids the error prone counting of instructions for manually
programmed forward branches.

I've updated VM_Compiler and am in the process of updating the rest of the
PPC files that generate assembly code to remove the old style branches.

--dave