cmake: copy globals from libc_nano.a to z_libc_partition

When building with NEWLIB_LIBC_NANO we need to copy
the globals from libc_nano.a into application memory
partition z_libc_partition, exactly as we do for
libc.a globals. This is required so that these globals
are accessible by nPRIV code, when building with
user mode support.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>