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/_rpo.ssa

@@ -0,0 +1,12 @@
+function $test() {
+@start
+	jmp @foo
+@baz
+	jnz 1, @end, @foo
+@bar
+	jmp @end
+@foo
+	jnz 0, @bar, @baz
+@end
+	ret
+}