Use C++ Name Mangling.

Add a Platform interface to abstract the platform-specific code generation details.
	src/Compiler/CodeGen/IPlatform.h

Add an implementation of IPlatform for i386 on OS X to mangle method names.
	src/Compiler/CodeGen/Platform_i386_OSX.h
	src/Compiler/CodeGen/Platform_i386_OSX.cpp

Use the Platform_i386_OSX class to replace the get_mangled() function.
	src/code_gen.cpp

Update the Compiler::AST::Method instance in the methodRef vector when we find one that has been overridden by a subclass since we no longer consult the string table to do method lookup.
	src/type_check3.cpp
Remove PowerPC Code Generation.

Remove PowerPC code generation as I no longer have any hardware to test it on.
	src/code_gen.h
	src/code_gen.cpp
	src/code_gen_ppc.cpp

There's no PowerPC architecture to parse so remove that dead code.
	src/minijava.cpp

And we don't need the PowerPC language bridging code.
	src/mj_dispatch.c