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/_fix2.ssa

@@ -0,0 +1,15 @@
+function $test() {
+@start
+	%x =w copy 1
+@loop
+	jnz %x, @noz, @isz
+@noz
+	%x =w copy 0
+	jnz %x, @loop, @end
+@isz
+	%x =w copy 1
+	jmp @loop
+@end
+	%z =w add 10, %x
+	ret
+}