cbe: legalize safety instructions in non-zig1 builds

This is valid if the bootstrap dev env doesn't need to support runtime
safety.  Another solution can always be implemented if needs change.

src/dev.zig

@@ -1,5 +1,8 @@
 pub const Env = enum {
     /// zig1 features
+    /// - `-ofmt=c` only
+    /// - `-OReleaseFast` or `-OReleaseSmall` only
+    /// - no `@setRuntimeSafety(true)`
     bootstrap,
 
     /// zig2 features