Redefine symbol TLS slot indices.

Redefine symbol TLS slot indices.

  * Instead of having symbol TLS slots be fixnum indices treat them as
byte offsets from the base of the thread structure.

  * This has no effect on the compiler or backends.

  * TLS slot allocation must now be done in terms of words, not fixnums.

  * In %{set-,}symbol-value-in-thread, use get-lisp-obj-address instead
of scaling the TLS index.

  * Use explicit WORD_SHIFT instead of make_fixnum() / fixnum_value() in
the runtime.