Avoid mpz for some common timestamp cases

Avoid mpz for some common timestamp cases

Performance problem reported by Gerd Möllmann and Mattias Engdegård in:
https://lists.gnu.org/r/emacs-devel/2024-06/msg00530.html
https://lists.gnu.org/r/emacs-devel/2024-06/msg00539.html
* src/timefns.c (CFORM_SECS_ONLY): The exact tv_nsec value is now
ignored if nonnegative (i.e., the only thing that matters is that
it’s nonnegative).
(decode_time_components): Use intmax_t instead of mpz arithmetic
if the tick count fits.  Add another ‘default: eassume (false);’
so that the revised code pacifies --enable-gcc-warnings with GCC
11.4.0 on x86-64.

(cherry picked from commit e30706fd12bce459b56123ad36a4e8c1c9d374e1)