wcc: tagged float-variant return packs float bits via X0-spill (#157)

The N_RETURN tagged-pack scalar-variant arm did MOVQ AX,DX, but a float
variant's value is in X0 not AX -> packed stale int (broke stof64/stof32
return (f64|invalid|overflow)). Fix: float variant bridges X0->DX via a
stack slot (SUBQ $8,SP; MOVQ $0,(SP); MOVSS|MOVSD X0,(SP); MOVQ (SP),DX;
ADDQ $8,SP), gated type_isfloat/exprfloatkind. No MOVQ-xmm->gp form
exists, hence the spill (715-class, cgreturn-register-pack twin of 715's
store-to-slot). Zero-slot-first -> deterministic f32 high-4. AX-independent
-> also resolves the multi-variant cs!=ww. Bootstrap-NEUTRAL (compiler has
no float-tagged-return). Test 707 +3 rows (f64/f32/multi, slot+8 bit-exact;
f32 no-f32-arg to isolate #143). Make test 184/184 incl 990-997 byte-id.

cmd/w6c/cgen.c

@@ -7716,6 +7716,35 @@ cgstmt(Cg *c, Node *n, Local **locals, int *frame)
 						    areg(D_CX));
 						ins2(c, A_MOVQ, areg(D_AX),
 						    areg(D_DX));
+					} else if (type_isfloat(vt)) {
+						/* #157: float variant. cgexpr left
+						 * the value in X0, not AX; there is
+						 * no MOVQ-xmm->gp encoding, so bridge
+						 * X0->DX through a stack slot (same
+						 * SUBQ/MOVSD/ADDQ idiom as the arg-
+						 * push at cgen.c:5367). Zero the slot
+						 * first so the f32 case (MOVSS writes
+						 * only the low 4 bytes) leaves a
+						 * deterministic high-4 — cs==ww byte-
+						 * id, matching f64's MOVSD which fills
+						 * all 8. The AX-independent spill also
+						 * removes the stale-AX cs!=ww on
+						 * multi-variant returns. */
+						int isf32 = type_isf32(vt);
+						ins2(c, A_SUBQ, aimm(8), areg(D_SP));
+						ins2(c, A_MOVQ, aimm(0),
+						    amem(D_SP, 0));
+						ins2(c, isf32 ? A_MOVSS : A_MOVSD,
+						    areg(D_X0), amem(D_SP, 0));
+						ins2(c, A_MOVQ, amem(D_SP, 0),
+						    areg(D_DX));
+						ins2(c, A_ADDQ, aimm(8), areg(D_SP));
+						if (rsz > 16)
+							ins2(c, A_MOVQ, aimm(0),
+							    areg(D_CX));
+						if (rsz > 24)
+							ins2(c, A_MOVQ, aimm(0),
+							    areg(D_R8));
 					} else {
 						ins2(c, A_MOVQ, areg(D_AX),
 						    areg(D_DX));