0.8.20.23:

0.8.20.23:
	Fix some GC problems introduced in the x86-64 immediate single-floats
        commit:
        * Make gencgc understand immediate single-floats too.
        * Fix typo in purify.

        Test changes:
        * Don't test floating-point overflow detection on x86-64, add
          a BUGS entry.
        * Change #-(x86 linux) to #-(and x86 linux) in debug.impure.lisp.