Fix two bugs in floating-point environment.

Fix two bugs in floating-point environment.

1. Don't cache it in the current thread.

   We can't mark everywhere the cache needs to be invalidated --
   i.e., every floating-point instruction -- and it's not clear
   there's any performance benefit to the cache anyway.  The main
   performance cost, as I recall, was swapping environments on every
   thread switch, which we avoid for all threads in the default
   environment.

2. The default environment initialization left the machine in a wacky
   state after reset-package!, which caused many spurious exception
   traps once I undid the cache.  There's no need to preserve the
   machine environment here; we are setting up the default
   environment, after all, so the environment we're in when done
   should be the default one.