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 262989, 263210, 263230, 263231, 263239, 263242, 263243,

Export _libc_arm_fpu_present as a private symbol to be used by other
system libraries, for example libm.

On armv6 access both the softfloat and, when available, the vfp to get and
set the floating-point environment.

Build fenv-vfp.c with the softfp float abi. Without this gcc generates an
incorrect assembly file that doesn't allow for vfp instructions.

Only build the vfp/softfp switching code on armv6 as we don't support vfp
on anything earlier than this. This should fix the armeb and arm builds
when using gcc.

Add an optimised version of the float and double helper functions.

Add an optimised version of the float and double helper functions. These
are only used on armv6 when the vfp unit is detected. They will also be
available for the upcoming armv6hf platform, however while not used by
default there will need to be defined for any software that calls them
directly.