Merge init_private_section_data with copy_private_section_data

Merge init_private_section_data with copy_private_section_data

init_private_section_data is used by the linker and is a special case
of copy_private_section_data that copies a reduced set of section data
from input to output.  Merge the two functions, adding a link_info
param to copy_private_section_data and remove init_private_section_data.