Fix malfunctioning cursor display on 32-bit Gtk

Fix malfunctioning cursor display on 32-bit Gtk

This bug on 32-bit platforms was caused by the timespec_hz
definition going haywire because the C expression
FIXNUM_OVERFLOW_P (MOST_POSITIVE_FIXNUM) did not work in #if.
Eventually the numeric problem showed up as a malfunctioning
cursor (Bug#32992).  Fix the problem with MOST_POSITIVE_FIXNUM.
By the way, make_fixnum should check for integer overflow when
debugging; this would have made it easier to track this bug down.
But one fix at a time.
* src/lisp.h (INTTYPEBITS): Now a macro, so usable in #if.
(MOST_POSITIVE_FIXNUM): Mention it’s used in #if.