Keep input SHF_GNU_RETAIN sections and strip output SHF_GNU_RETAIN for GNU/FreBSD...

Keep input SHF_GNU_RETAIN sections and strip output SHF_GNU_RETAIN for GNU/FreBSD ELFOSABIs.

2020-12-14  H.J. Lu  <hjl.tools@gmail.com>
	    Cary Coutant  <ccoutant@gmail.com>

elfcpp/
	PR gold/27039
	* elfcpp.h (SHF): Add SHF_GNU_RETAIN.

gold/
    	PR gold/27039
	* layout.cc (Layout::layout): Strip SHF_GNU_RETAIN.
	* object.cc (Sized_relobj_file::Sized_relobj_file): Initialize osabi_.
	(Sized_relobj_file::do_layout): Keep SHF_GNU_RETAIN sections for
	GNU/FreBSD ELFOSABIs.
	* object.h (Osabi) New class.
	(Sized_relobj_file): Add osabi() and osabi_.
	* testsuite/Makefile.am (check_SCRIPTS): Add retain.sh.
	(check_DATA): Add retain_1.out retain_2.out.
	(MOSTLYCLEANFILES): Add retain_1 retain_2.
	(retain_1.out): New target.
	(retain_1): Likewise.
	(retain_1.o): Likewise.
	(retain_2.out): Likewise.
	(retain_2): Likewise.
	(retain_2.o): Likewise.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/retain.sh: New file.
	* testsuite/retain_1.s: Likewise.
	* testsuite/retain_2.s: Likewise.