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/_dragon.ssa

@@ -0,0 +1,33 @@
+# a moderately complex test for
+# dominators computation from
+# the dragon book
+# because branching is limited to
+# two, I had to split some blocks
+
+function $dragon() {
+@start
+@b1
+	jnz 0, @b2, @b3
+@b2
+	jmp @b3
+@b3
+	jmp @b4.1
+@b4.1
+	jnz 0, @b3, @b4.2
+@b4.2
+	jnz 0, @b5, @b6
+@b5
+	jmp @b7
+@b6
+	jmp @b7
+@b7
+	jnz 0, @b8.1, @b4.1
+@b8.1
+	jnz 0, @b3, @b8.2
+@b8.2
+	jnz 0, @b9, @b10
+@b9
+	jmp @b1
+@b10
+	jmp @b7
+}