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/env.ssa

@@ -0,0 +1,21 @@
+# sanity checks for env calls
+
+function l $epar(env %e, l %i) {
+@start
+	%x =l add %e, %i
+	ret %x
+}
+
+export function l $earg(l %a, l %b) {
+@start
+	%r1 =l call $epar(env %a, l %b)
+	# okay to call a regular function
+	# with an env argument
+	%r2 =l call $labs(env 113, l %r1)
+	ret %r2
+}
+
+# >>> driver
+# extern long earg(long, long);
+# int main(void) { return !(earg(2, -44) == 42); }
+# <<<