Add a new ARM TARGET_ARCH, armv6hf. This is considered experimental.

This targets the existing ARMv6 and ARMv7 SoCs that contain a VFP unit.
This is an optional coprocessors may not be present in all devices, however
it appears to be in all current SoCs we support.

armv6hf targets the VFP variant of the ARM EABI and our copy of gcc is too
old to support this. Because of this there are a number of WITH/WITHOUT
options that are unsupported and must be left as the default value. The
options and their required value are:
 * WITH_ARM_EABI
 * WITHOUT_GCC
 * WITHOUT_GNUCXX

In addition, without an external toolchain, the following need to be left
as their default:
 * WITH_CLANG
 * WITH_CLANG_IS_CC

As there is a different method of passing float and double values to
functions the ABI is incompatible with existing armv6 binaries. To use
this a full rebuild of world is required. Because no floating point values
are passed into the kernel an armv6 kernel with VFP enabled will work with
an armv6hf userland and vice versa.