Fix handling of switch to work properly
- Switch to emitting jump table in-line.  Previously, it was put in the RO section and thus not accessible by PC relative addressing.
- Dispatch switch by loading the table entry into PC (optimization)
- Add a new instruction form to load the address of the jump table into a register by adding a constant to the PC. This required adding a new fixup type that can adjust the immediate field of a format B instruction.
- Cleanup: remove VK_ fixup enumerations in MCExpr.  These don't seem to be needed.