Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r257207, r261161, r261163:

Update the hard-float version of the fenv functions to use the VFP unit.
Any other floating-point unit is unsupported on ARM.

Use __fenv_static for all static inline functions.

Correctly shift the mask when masking/unmasking exceptions.

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add an implementation of fenv.h intended for platforms that lack an FPU and
use softfloat.

Thanks to Ian Lepore for testing and debugging this patch. The fenv
regression tests pass (at least for Ian's arm chip) with this change.