cappuccino/ctrl: remove obsolete last_branch_target_pc calculation

With branches resolved in decode stage, looking at the
branch instruction in execute isn't meaningful anymore.
cappuccino/ctrl: whitespace fix
cappuccino/ctrl: remove tt & pic exception prevention on branches

The issues that those preventions solved are no longer
valid.
cappuccino/ctrl: update comment about branch signal
cappuccino/ctrl: remove commented out code
cappuccino/ctrl: fix delay slot handling for syscall exception

l.sys in delay slots are disallowed by the or1k spec,
but the value that epcr should take when this happens is
still defined as:
"Address of just executed jump instruction"

So let's just do what the spec says without any trickery.