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/cmp1.ssa

@@ -0,0 +1,17 @@
+# test cmp used in jnz as well as its result value
+
+export
+function w $test(w %c) {
+@start
+	%cmp =w cultw 1, %c
+	jnz %cmp, @yes, @no
+@yes
+	%cmp =w copy 1
+@no
+	ret %cmp
+}
+
+# >>> driver
+# int test(int);
+# int main(void) { return test(0); }
+# <<<