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/sum.ssa

@@ -0,0 +1,32 @@
+# Simple test for addressing modes.
+
+export
+function w $sum(l %arr, w %num) {
+@start
+@loop
+	%n1 =w phi @start %num, @loop1 %n2
+	%s0 =w phi @start 0, @loop1 %s1
+	%n2 =w sub %n1, 1
+	%c =w cslew %n1, 0
+	jnz %c, @end, @loop1
+@loop1
+	%idx0 =l extsw %n2
+	%idx1 =l mul 4, %idx0
+	%idx2 =l add %idx1, %arr
+	%w =w loadw %idx2
+	%s1 =w add %w, %s0
+	jmp @loop
+@end
+	ret %s0
+}
+
+# >>> driver
+# extern int sum(int *, int);
+# int arr[] = { 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21 };
+# #define N sizeof arr / sizeof arr[0]
+# int main() {
+# 	int i, s;
+# 	for (s=i=0; i<N; i++) s+=arr[i];
+# 	return !(sum(arr, N) == s);
+# }
+# <<<