elf_backend_finish_dynamic_symbol returning false

elf_backend_finish_dynamic_symbol returning false

Returning false from elf_backend_finish_dynamic_symbol will not result
in an error being printed unless bfd_error is set but will result in
the linker exiting with a non-zero status.  If just bfd_error is set
then a generic "final link failed" will result, which doesn't help a
user much.  So elf_backend_finish_dynamic_symbol should print its own
error message whenever returning false, or use BFD_ASSERT or abort to
print assertion failures for conditions that shouldn't occur.

This patch does that, and removes unnecessary "htab != NULL" tests in
elf_backend_finish_dynamic_symbol.  Such tests aren't needed in a
function only called via elf_backend_data.