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/_spill1.ssa

@@ -0,0 +1,22 @@
+# test with NReg == 3
+# there must be a spill
+# happening on %c
+#
+# if you replace the sub
+# by an add or comment
+# the two marked lines
+# there should be no
+# spill
+#
+
+function $test() {
+@start
+	%f =w copy 0      # here
+	%b =w copy 1
+	%c =w copy 2
+	%a =w sub %b, %c
+	%d =w copy %b
+	%e =w copy %f     # and there
+	%g =w copy %a
+	ret
+}