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/eucl.ssa

@@ -0,0 +1,25 @@
+# euclide's algorithm in ssa
+# it is a fairly interesting
+# ssa program because of the
+# swap of b and a
+
+export
+function $test() {
+@start
+
+@loop
+	%a =w phi @start 380, @loop %r
+	%b =w phi @start 747, @loop %a
+	%r =w rem %b, %a
+	jnz %r, @loop, @end
+
+@end
+	storew %a, $a
+	ret
+}
+
+# >>> driver
+# extern void test(void);
+# int a;
+# int main() { test(); return !(a == 1); }
+# <<<