0.8.17.8: x86 backtraces

0.8.17.8: x86 backtraces
           * fixed bug #345: don't set the escaped flag to NIL on x86
              in COMPUTE-CALLING FRAME if we have no code from
              FIND-ESCAPED-FRAME. Add tag-bytes to undefined_tramp and
              closure_tramp on x86 like others.
           * partial fix for #61: instead of throwing no-debug-info
              conditions, return a "no debug information for frame"
              bogus-debug-fun from debug-fun-from-pc.
           * Two new BUGS entries about backtraces.
           * Tests. (debug.impure.lisp now passes on sparc as well,
              but only because one test now axxepts the XEP in lieu of
              the function on sparc and x86.