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

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

2 years, 10 months, 22 days after the projects/arm_eabi branch was created
make the ARM EABI the default ABI on arm, armeb, armv6 and armv6eb.

This is intended to be the default ABI from now on with the old ABI to be
retired. Because of this all users are strongly suggested to upgrade to the
ARM EABI.

As the two ABIs are incompatible it is unlikely upgrading in place will
work. Users should perform a full backup and either use an external machine
to upgrade, or install to an alternative location on their media. They
should also reinstall all ports or packages when these are available.

The only known issues are:
- pkg incorrectly detects the ABI. This is fixed upstream, and will a
patch will be made to the port.
- GDB can have issues with executables built with clang.

__FreeBSD_version has been bumped.

Add compiler support for the ARM EABI.

ARM EABI support is disabled by default and can be enabled by setting
WITH_ARM_EABI when building, however only the kernel-toolchain target will
work with this flag until the rest of the support is added.