Allow target to add custom dynamic table entries.

Allow target to add custom dynamic table entries.

2014-03-10  Sasa Stankovic  <Sasa.Stankovic@imgtec.com>

gold/
	* output.cc (Output_data_dynamic::Dynamic_entry::write):
	Get the value of DYNAMIC_CUSTOM dynamic entry.
	* output.h (Output_data_dynamic::add_custom): New function.
	(Dynamic_entry::Dynamic_entry): New constructor for DYNAMIC_CUSTOM
	dynamic entry.
	(enum Dynamic_entry::Classification): Add DYNAMIC_CUSTOM.
	* target.h (Target::dynamic_tag_custom_value): New function.
	(Target::do_dynamic_tag_custom_value): New function.