Fix GC bug with Lisp floats and --with-wide-int

Fix GC bug with Lisp floats and --with-wide-int

On --with-wide-int platforms where Lisp_Object can be
put into non-adjacent registers, mark_maybe_pointer failed
to mark a float whose only reference was as a tagged pointer.
* src/alloc.c (live_float_holding): New function,
a generalization of the old live_float_p.
(live_float_p): Use it.
(mark_maybe_pointer): Use live_float_holding, not live_float_p.