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/_spill2.ssa

@@ -0,0 +1,22 @@
+# stupid spilling test
+
+function $test() {
+@start
+	%x1  =w copy 10
+	%x2  =w add %x1, %x1
+	%x3  =w sub %x2, %x1
+	%x4  =w add %x3, %x1
+	%x5  =w sub %x4, %x1
+	%x6  =w add %x5, %x1
+	%x7  =w sub %x6, %x1
+	%x8  =w add %x7, %x1
+	%x9  =w sub %x8, %x8
+	%x10 =w add %x9, %x7
+	%x11 =w sub %x10, %x6
+	%x12 =w add %x11, %x5
+	%x13 =w sub %x12, %x4
+	%x14 =w add %x13, %x3
+	%x15 =w sub %x14, %x2
+	%x16 =w add %x15, %x1
+	ret
+}