Add ARM version detection to configure and use the resulting macros.

This defines ARM_VERSION_{5,6,7}. Several of these can be
defined at once. For example ARM_VERSION_5 and ARM_VERSION_6
will both be defined if the target is an ARM v6 CPU.

Previously, we defined HAVE_ARMV6 based on running a program.
This obviously didn't work for cross-compilation. This new
approach does, and detects more cases (v5, v6, v7) than the
previous check (v6).