Fix internal error with LTO on ARM.

Fix internal error with LTO on ARM.

This prevents the target-specific do_read_symbols methods from being called
twice when do_layout_deferred_sections needs to layout an .eh_frame section.

gold/
	PR gold/15639
	* dynobj.h (Sized_dynobj::base_read_symbols): New method.
	* dynobj.cc (Sized_dynobj::do_read_symbols): Move body to...
	(Sized_dynobj::base_read_symbols): ...new method.
	* object.h (Sized_relobj_file::base_read_symbols): New method.
	* object.cc (Sized_relobj_file::do_read_symbols): Move body to...
	(Sized_relobj_file::base_read_symbols): ...new method.
	* arm.cc (Arm_relobj::do_read_symbols): Call base_read_symbols.
	* mips.cc: (Mips_relobj::do_read_symbols): Likewise.
	* powerpc.cc (Powerpc_dynobj::do_read_symbols): Likewise.