Fix symbol-value-in-thread GC race condition.

Fix symbol-value-in-thread GC race condition.

  * SVIT was using SAP-REF-WORD and MAKE-LISP-OBJ while the GC was
enabled, which is mostly-safe on x86oids, but technically breaks
the consistency rules for the GC.  Failures due to badly-timed GC
(well-timed GC?) have been observed on PPC.

  * Instead of SAP-REF-WORD, checking for specific tags, then
using MAKE-LISP-OBJ and checking for validity and GC epoch, which
is a badly-broken approach, use SAP-REF-LISPOBJ to obtain the
value and then check the tags with GET-LISP-OBJ-ADDRESS, a far
safer approach, with lower overhead, and not subject to random GC
lossage.