[PATCH] Implement __builtin_issignaling

[FX]

Hi Jakub,

Thank you for taking this on (this patch and the Fortran ones), it is really appreciated and will improve gfortran's IEEE conformance.

- "Implement __builtin_issignaling”: is approved for the tiny Fortran part
- "libgfortran: Use __builtin_issignaling in libgfortran”: was approved by Thomas
- I will have to look at the next two patches (ieee_value and ieee_class) in the next few days. I think we can make adjustments to the runtime library as well.

Related: there are several tests in our testsuite that have { dg-require-effective-target issignaling }
This is checked by check_effective_target_issignaling in gcc/testsuite/lib/target-supports.exp, and probes the support for the issignaling macro in <math.h>. I think it would make sense to adjust those tests to run on a wider range of targets, using the new built-in.

FX