Fix x86-64 backend bugs found using Paul Dietz's random tester.
* Sign-extension in constant LOGAND, + and TRUNCATE VOPs.
* Sign-extension of literal (unsigned-byte 32) passed as
arguments on the stack.
Fix handling of :START1 and :START2 in the string comparison
deftransforms on simple-base-strings (ansi-tests MISC.572/573/574).
Minor cleanup: Use the already defined *cache-expand-threshold*
instead of magic numbers in pcl/cache.lisp.