[lld] handle the case of errors from createLinkerInput

This changes the interface of createLinkerInput to use ErrorOr, so that
errors from the linker can be captured.

Also adds a convenience function for error strings to be returned from
file nodes.
[lld][ELF][Hexagon] add Init/Fini option for hexagon

This adds init/fini support for Hexagon.
[lld][ELF] Change the InitFini atom type from Stub to Data
[lld][ELF][X86_64] Fix alignment of init_array/fini_array sections