Bugfix#70896 gmp_fact() silently ignores non-integer inputs

Bugfix#70896 gmp_fact() silently ignores non-integer inputs

Factorials only make sense for integer inputs.
To do something factorial-like, the Gamma Function
should be used instead.
However, at this point it's no longer a factorial.

For PHP/GMP, we'll raise a warning on trying to use
a non-integer input, but carry on returning the truncated
value as we used to (avoiding BC breakage).