[Patch, Fortran] PR57496 use finiteq etc. for __float in write_float.def

[Tobias Burnus]

Janne has approved the patch on IRC, hence, I committed it as Rev. 199598.

Thus, the REAL(16) issue is now solved.

However, REAL(10) still fails - and has still to be debugged. See PR.

Tobias