bug#32463: 27.0.50; (logior -1) => 4611686018427387903

[Paul Eggert]

> Have you checked a mini-gmp build to ensure that this patch works if
> the GMP library is not installed ?

I have now. :-)  It works, in the sense that Emacs builds and fails only the 
tests that it was already failing.

> It might be slightly faster to use mpz_limbs_read in make_number instead
> of mpz_getlimbn.

On Fedora 28 x86-64, mpz_getlimbn is inline whereas mpz_limbs_read is a function 
call, so it's not clear which should be faster. Another item on the list of 
things to do....