[hw] Pop a scalar FP result from FPU0 only

[emiliengnr]

Hello, here is a pull request for a bug I found.

Problem

A scalar FP instruction followed by a vector FP instruction wedges the VFU, and before it wedges it
drops one VRF word of the vector result.

Scalar instructions go to fpnew lane 0 only (spatz_vfu.sv:1576):

assign int_fpu_in_valid = spatz_req_valid && operands_ready && (!spatz_req.op_arith.is_scalar || fpu == 0) && is_fpu_insn;

The output handshake has no matching term (spatz_vfu.sv:1630), out_ready_i is the shared
result_ready. It is computed from lane 0's tag, and for a scalar result pending_results covers
the low ELEN/8 bytes, so the gate closes on lane 0 alone and every lane pops. Lanes 1 to N-1 hold
the first word of the vector instruction that followed, and that word is lost. Lane 0 then trails by
one result. When the vector op reaches its last word only lane 0 holds it, the all-lane gate
&(result_valid | ~pending_results) can no longer close, and the write-back, the drain and the
response emit all stop. fpnew holds busy_o high on the unpopped result, so the FSM cannot leave
VFU_RunningFPU and the core hangs on the next dependent instruction.

A lane probe on a default build (n_fpu=4, ELEN=64), scalar VFADD at vl=1 then vector
VFMINMAX at vsew=EW_64 vl=8, otid the tag id at each lane output, LSB lane first:

rvalid=ffffffff rdy=1 gate=1 tag[id=0 wb=1 last=1] otid=0,2,2,2   scalar pop, lanes 1-3 lose vector word 0
rvalid=ffffffff rdy=1 gate=1 tag[id=2 wb=0 last=0] otid=2,2,2,2   mixed word write-back
rvalid=000000ff rdy=0 gate=0 tag[id=2 wb=0 last=1] otid=2,0,0,0   wedge, lane 0 alone holds the last word

Fix

Give the output handshake the lane term the input already has:

// Only the FPU0 retires scalar instructions
logic int_fpu_out_ready;
assign int_fpu_out_ready = result_ready && (!result_tag.wb || fpu == 0);

wired to .out_ready_i. Lane 0 drains its scalar result on its own and presents the vector word one
cycle later, then all lanes pop it together. Vector instructions are unaffected, their
result_tag.wb is 0.