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/minic/mcc

@@ -0,0 +1,44 @@
+#!/bin/sh
+
+DIR=`cd $(dirname $0); pwd`
+QBE=$DIR/../qbe
+
+usage()
+{
+	echo "usage: mcc [LDFLAGS] file.c" >&2
+	exit 1
+}
+
+for i
+do
+	case $i in
+	-*)
+		flags="$flags $i"
+		;;
+	*)
+		if ! test -z $file
+		then
+			usage
+		fi
+		file=$i
+		;;
+	esac
+done
+
+if test -z $file
+then
+	usage
+fi
+
+
+$DIR/minic < $file          > /tmp/minic.ssa &&
+$QBE       < /tmp/minic.ssa > /tmp/minic.s   &&
+cc /tmp/minic.s $flags
+
+if test $? -ne 0
+then
+	echo "error processing file $file" >&2
+	exit 1
+fi
+
+