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/prime.ssa

@@ -0,0 +1,33 @@
+# find the 10,001st prime
+# store it in a
+
+export
+function $test() {
+@start
+@loop
+	%n =w phi @start 5, @tloop %n, @yes %n1
+	%p =w phi @start 13, @tloop %p1, @yes %p1
+	%p1 =w add %p, 2
+@tloop
+	%t =w phi @loop 3, @next %t1
+	%r =w rem %p, %t
+	jnz %r, @next, @loop
+@next
+	%t1 =w add 2, %t
+	%tsq =w mul %t1, %t1
+	%c0 =w csgtw %tsq, %p
+	jnz %c0, @yes, @tloop
+@yes
+	%n1 =w add 1, %n
+	%c1 =w ceqw 10001, %n1
+	jnz %c1, @end, @loop
+@end
+	storew %p, $a
+	ret
+}
+
+# >>> driver
+# extern void test(void);
+# int a;
+# int main() { test(); return !(a == 104743); }
+# <<<