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/fixarg.ssa

@@ -0,0 +1,15 @@
+# regression test for 3bec2c60
+
+export
+function w $test() {
+@start
+	%x1 =l alloc8 8
+	%x2 =l alloc8 8
+	%r =w cnel %x1, %x2  # both operands need fixing
+	ret %r
+}
+
+# >>> driver
+# extern int test();
+# int main() { return !(test() == 1); }
+# <<<