devel/arm-none-eabi-newlib: automatically generate pkg-plist

Now that devel/arm-none-eabi-gcc has options for different multilib
layouts, the generated tree and thus pkg-plist heavily depends on the
actual multilib option applied.

It's thus best to automatically create the plist now.

Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D32755