ARM: kernel: Make 32/64-bit atomics work for ARMv5/6

Support for 64-bit atomic operations for ARMv7+ is currently stubbed
out in libroot, but our current targets do not use it anyway.

We now select atomics-as-syscalls automatically based on the ARM
architecture we're building for. The intent is to do away with
most of the board specifics (at the very least on the kernel side)
and just specify the lowest ARMvX version you want to build for.

This will give flexibility in being able to distribute a single
image for a wide range of devices, and building a tuned system
for one specific core type.