Clean up handling of JTOC

Some code still assumes that the JTOC is not a constant. This adds noise to the code and creates confusion; as well, it can be an impediment when adding new code. In this improvement I will clean up the JTOC situation as much as possible, making it so that the native code assumes that it is a constant value that it acquires immediately upon loading, while the Java code has it as a baked-in constant. Notably, the baseline compiler on IA32 will no longer generate code to load the JTOC from the VM_Processor, and the VM_Processor will no longer have a JTOC field.