[fortran,patch] Handle infinities and NaNs in intrinsics code generation

[FX]

ping

> After the compile-time simplification, this patch fixes the handling of 
> special values (infinities and NaNs) by intrinsics EXPONENT, FRACTION, 
> SPACING, RRSPACING & SET_EXPONENT on the code generation side.
> 
> Bootstrapped and regtested on x86_64-linux.
> OK to commit?

intrinsics.ChangeLog
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intrinsics.diff
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