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/vararg1.ssa

@@ -0,0 +1,35 @@
+export
+function d $f(l %x, ...) {
+@start
+	%vp =l alloc8 32
+	vastart %vp
+	%i =l vaarg %vp
+	%n =d vaarg %vp
+	ret %n
+}
+
+export
+function w $g(l %fmt, ...) {
+@start
+	%vp =l alloc8 32
+	vastart %vp
+	%r =w call $print(l %fmt, l %vp)
+	ret %r
+}
+
+# >>> driver
+# #include <stdarg.h>
+# #include <stdio.h>
+# extern double f(int, ...);
+# extern int g(char *, ...);
+# int print(const char *fmt, va_list *ap) {
+# 	return vprintf(fmt, *ap);
+# }
+# int main() {
+# 	g("Hell%c %s %g!\n", 'o', "world", f(42, "x", 42.0));
+# }
+# <<<
+
+# >>> output
+# Hello world 42!
+# <<<