[PATCH v2][PR libgfortran/78314] Fix ieee_support_halting

[Szabolcs Nagy]

ieee_support_halting only checked the availability of status
flags, not trapping support.  On some targets the later can
only be checked at runtime: feenableexcept reports if
enabling traps failed.

So check trapping support by enabling/disabling it.

Updated the test that enabled trapping to check if it is
supported.

Tested on aarch64-linux-gnu and x86_64-linux-gnu.

gcc/testsuite/
2016-11-16  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	PR libgfortran/78314
	* gfortran.dg/ieee/ieee_6.f90: Use ieee_support_halting.

libgfortran/
2016-11-16  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	PR libgfortran/78314
	* config/fpu-glibc.h (support_fpu_trap): Use feenableexcept.

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