libc: Make libc-nano default for ARM toolchain if newlib

The ARM embedded toolchain has 2 newlib based libc build variants, one
that utilizes the "nano" configuration which is more in line with the
Zephyr SDK.  Make the "nano" cfg the default if newlib is enabled to
match closer how the Zephyr SDK behaves.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>