cappuccino/lsu: disconnect external bus from lsu_valid_o

this will introduce a 1 cycle latency for non-cache
accesses, but lsu_valid_o is connected to execute_valid,
which is a very critical signal.
execute_alu: seperate a_lt_b into seperate signed/unsigned signals

This removes the chain of two muxes for decoding
opc_alu_secondary, smart tools can maybe figure this out,
but this makes it explicit to help them on the way if
they can't.
cappuccino/ctrl: fix one-off in SPR_GPR0 spr sys group read