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/loop.ssa

@@ -0,0 +1,24 @@
+# simple looping program
+# sums all integers from 100 to 0
+
+export
+function $test() {
+@start
+
+@loop
+	%s  =w phi @start   0, @loop %s1
+	%n  =w phi @start 100, @loop %n1
+	%s1 =w add %s, %n
+	%n1 =w sub %n, 1
+	jnz %n1, @loop, @end
+
+@end
+	storew %s1, $a
+	ret
+}
+
+# >>> driver
+# extern void test(void);
+# int a;
+# int main() { test(); return !(a == 5050); }
+# <<<