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/test/align.ssa

@@ -0,0 +1,17 @@
+export
+function $test() {
+@start
+	%x =l alloc16 16
+	%y =l add %x, 8
+	%m =w rem %y, 16
+	storew %m, %y
+	%n =w loadw %y
+	storew %n, $a
+	ret
+}
+
+# >>> driver
+# extern void test(void);
+# int a;
+# int main() { test(); return !(a == 8 || a == -8); }
+# <<<