GAS: Update the .section directive so that a numeric section index can be provided...

GAS: Update the .section directive so that a numeric section index can be provided when the "o" flag is used.

	PR 26253
gas	* config/obj-elf.c (obj_elf_section): Accept a numeric value for
	the "o" section flag.  Interpret it as a section index.  Allow an
	index of zero.
	* doc/as.texi: Document the new behaviour.
	* NEWS: Mention the new feature.  Tidy entries.
	* testsuite/gas/elf/sh-link-zero.s: New test.
	* testsuite/gas/elf/sh-link-zero.d: New test driver.
	* testsuite/gas/elf/elf.exp: Run the new test.
	* testsuite/gas/elf/section21.l: Updated expected assembler
	output.

bfd	* elf.c (_bfd_elf_setup_sections): Do not complain about an
	sh_link value of zero when the SLF_LINK_ORDER flag is set.
	(assign_section_numbers): Likewise.