Cosmetic improvements for OSABI access.

Cosmetic improvements for OSABI access.

Add accessor methods to elfcpp::Ehdr class for EI_OSABI and EI_ABIVERSION;
use those to simplify initialization of Osabi class and eliminate the need
to template the class.

elfcpp/
	* elfcpp.h (class Ehdr): Add get_ei_osabi and get_ei_abiversion methods.

gold/
	* dwp.cc (class Dwo_file): Use new Ehdr::get_ei_osabi and
	get_ei_abiversion methods.
	* incremental.cc (make_sized_incremental_binary): Likewise.
	* object.cc (Sized_relobj_file::Sized_relobj_file): Likewise.
	(make_elf_sized_object): Likewise.
	* object.h (class Osabi): Make the class untemplated.