Fix bug in jump instructions

* As stated in earlier commits we prefetch next instruction before
the current one gets executed to guarantee that we have it available
in case a branch instruction changes the PC. So a typical fetch cycle
for a branch instruction would look like:

/* Cycle 1. */
instr = <something>
next_instr = read(PC) -> jump
PC += 4 (now it points to the branch delay)

/* Cycle 2. */
instr = jump
next_instr = read(PC) -> branch delay
PC += 4 (now it points to the instruction AFTER the delay slot)

<execute branch>

So if a branch uses offsets instead of hard coding the PC, it will point
to the wrong address since it expects to have the PC pointing to the branch
delay instruction. To fix this, subtrack 4 from the pc.