w6c: global *struct base field store loads the pointer from g(SB)

cstage dereferenced 0(BP) for the base — a SEGV on every store
through a module-global struct pointer; wwstage was already correct
(the inverse-set member). Mirror ww's global-pointer load. New
989_globptrfield_run pins cs==ww both stages. Task #47.

cmd/w6c/cgen.c

@@ -5368,7 +5368,27 @@ cgexpr(Cg *c, Node *n, Local *locals)
 				}
 				/* now store AX into target */
 				if (via_ptr) {
-					ins2(c, A_MOVQ, amem(D_BP, boff), areg(D_BX));
+					if (boff == 0 && let_islet(base->str)) {
+						/* #47 (inverse): a GLOBAL *struct
+						 * base. The pointer VALUE lives at
+						 * gp(SB), not a BP slot — load it
+						 * from the symbol. Pre-fix MOVQ
+						 * (BP),BX derefed saved BP → SEGV.
+						 * Mirrors the already-correct wwstage
+						 * global-base load; cs==ww. (Other
+						 * via_ptr field arms keep the latent
+						 * BP-deref — inverse siblings #60/#61,
+						 * ken-triage.) */
+						ins1(c, A_PUSHQ, areg(D_AX));
+						ins2(c, A_LEAQ,
+						    masym(c, base->str), areg(D_BX));
+						ins2(c, A_MOVQ, amem(D_BX, 0),
+						    areg(D_BX));
+						ins1(c, A_POPQ, areg(D_AX));
+					} else {
+						ins2(c, A_MOVQ,
+						    amem(D_BP, boff), areg(D_BX));
+					}
 					ins2(c, store_op, areg(D_AX), amem(D_BX, foff));
 				} else if (is_global) {
 					ins2(c, A_LEAQ, masym(c, base->str), areg(D_BX));