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/abi2.ssa

@@ -0,0 +1,19 @@
+type :fps = { s, b, s }
+
+export
+function s $sum(:fps %p) {
+@start
+	%f1 =s load %p
+	%p8 =l add 8, %p
+	%f2 =s load %p8
+	%s =s add %f1, %f2
+	ret %s
+}
+
+# >>> driver
+# typedef struct { float f1; char b; float f2; } fps;
+# extern float sum(fps);
+# int main() { fps x = { 1.23, -1, 2.34 }; return !(sum(x) == 1.23f+2.34f); }
+# /* Note the f suffixes above are important
+#  * otherwise C does double operations. */
+# <<<