Welcome clang as the default compiler on ARM.

Change the default compiler for little-endian ARM to clang to allow for more
testing before 10.0 is released. As LLVM and clang currnetly lack support
for big-endian ARM leave gcc as the default there.

This will also allow us to investigate moving to use the hard floating-point
version of the ARM EABI on SoCs that include the Vector Floating Point unit.
A version of this is included in all ARMv6 and ARMv7 SoCs we currently, and
are likely to support.

Both the current ABI and the new EABI are supported by clang and it will be
built correctly depending on which is selected by the user.