Implement tcall pseudo-instruction

[kevinhartman]

This one will be a little bit more interesting, since this should cause proper code for the target CPU to be generated.

One way to do this would be to let tcall instructions fall through from the pseudo instruction handler (don't handle, but probably log the call), and expect that each AssemblerTarget handler will handle tcall.

Another approach would be to have a separate method on AssemblerTarget for generating tcall code, which the assembler core could attach to the pseudo instruction handler (i.e. function pointer in pseudo instruction handler's constructor).