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/_live.ssa

@@ -0,0 +1,21 @@
+# this control flow graph is irreducible
+# yet, we expecet the liveness analysis
+# to work properly and make %x live in
+# the block @left
+#
+# nothing should ever be live at the entry
+
+function $test() {
+@start
+	%b =w copy 0
+	%x =w copy 10
+	jnz 0, @loop, @left
+@left
+	jmp @inloop
+@loop
+	%x1 =w add %x, 1
+@inloop
+	%b1 =w add %b, 1
+@endloop
+	jmp @loop
+}