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/tools/cra.sh

@@ -0,0 +1,38 @@
+#!/bin/sh
+
+DIR=`cd $(dirname "$0"); pwd`
+QBE=$DIR/../qbe
+BUGF=/tmp/bug.id
+FIND=$1
+FIND=${FIND:-afl-find}
+
+if ! test -f $BUGF
+then
+	echo 1 > $BUGF
+fi
+
+while true
+do
+	ID=`cat $BUGF`
+
+	if test `ls $FIND/crashes/id* | wc -l` -lt $ID
+	then
+		rm -f bug.ssa
+		echo "All done!"
+		exit 0
+	fi
+
+	BUG=`ls $FIND/crashes/id* | sed -ne "${ID}{p;q}"`
+
+	echo "*** Crash $ID"
+	cp $BUG bug.ssa
+
+	$QBE bug.ssa > /dev/null
+	RET=$?
+	if test \( $RET -ne 0 \) -a \( $RET -ne 1 \)
+	then
+		exit 1
+	fi
+
+	expr $ID + 1 > $BUGF
+done