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/cup.ssa

@@ -0,0 +1,18 @@
+# counts up from -1988 to 1991
+
+export
+function $test() {
+@start
+@loop
+	%n0  =l phi @start -1988, @loop %n1
+	%n1  =l add 1, %n0
+	%cmp =w cslel 1991, %n1
+	jnz %cmp, @end, @loop
+@end
+	ret
+}
+
+# >>> driver
+# extern void test(void);
+# int main() { test(); return 0; }
+# <<<