[GOLD] OSABI not set when STT_GNU_IFUNC or STB_GNU_UNIQUE symbols output

[GOLD] OSABI not set when STT_GNU_IFUNC or STB_GNU_UNIQUE symbols output

This patch arranges to have OSABI set to ELFOSABI_GNU (if not set to
some other non-zero value) when gold outputs an ifunc local or global
symbol, or a unique global symbol to either .dynsym or .symtab.
STT_GNU_IFUNC and STB_GNU_UNIQUE have values in the LOOS to HIOS range
and therefore require interpretation according to OSABI.

I'm not sure why parameters->target() is const Target& while
parameters->sized_target() is Sized_target*, but it's inconvenient to
use the latter in Symbol_table::finalize.  So this patch adds another
const_cast complained about in layout.cc and gold.cc.

	PR 24853
	* symtab.h (set_has_gnu_output, has_gnu_output_): New.
	* symtab.cc (Symbol_table::Symbol_table): Init has_gnu_output_.
	(Symbol_table::finalize): Set ELFOSABI_GNU when has_gnu_output_.
	(Symbol_table::set_dynsym_indexes, Symbol_table::sized_finalize):
	Call set_has_gnu_output for STT_GNU_IFUNC and STB_GNU_UNIQUE globals.
	* object.cc (Sized_relobj_file::do_finalize_local_symbols): Call
	set_has_gnu_output when STT_GNU_IFUNC locals will be output.