PR 10450
* output.h (class Output_section): Add is_entsize_zero_ field.
* output.cc (Output_section::Output_section): Initialize
is_entsize_zero_.
(Output_section::set_entsize): If two different entsizes are
requested, force it to zero.
(Output_section::add_input_section): Set flags for .debug_str
before updating section flags. Set entsize.
(Output_section::update_flags_for_input_section): Set SHF_MERGE
and SHF_STRING if all input sections have those flags.