fix(qbe): any-child terminator check in QBE_IsTerminator for hBlock — QBE SSA now fully valid

- QBE_IsTerminator(hBlock) checks ALL children, not just last
- Removed legacy c_backend.bux and nim_backend.bux from _selfhost/src/
- QBE self-hosting QBE phase: PASSES (SSA valid, assembled to .s)
- Remaining: linker errors from missing String_Eq/StringBuilder in runtime

vendor/qbe/abi.c

@@ -0,0 +1,25 @@
+#include "all.h"
+
+/* eliminate sub-word abi op
+ * variants for targets that
+ * treat char/short/... as
+ * words with arbitrary high
+ * bits
+ */
+void
+elimsb(Fn *fn)
+{
+	Blk *b;
+	Ins *i;
+
+	for (b=fn->start; b; b=b->link) {
+		for (i=b->ins; i<&b->ins[b->nins]; i++) {
+			if (isargbh(i->op))
+				i->op = Oarg;
+			if (isparbh(i->op))
+				i->op = Opar;
+		}
+		if (isretbh(b->jmp.type))
+			b->jmp.type = Jretw;
+	}
+}