Add new directive to GAS: .attach_to_group.

Add new directive to GAS: .attach_to_group.

	* config/obj-elf (elf_pseudo_table): Add attach_to_group.
        (obj_elf_attach_to_group): New function.
        * doc/as.texi: Document the new directive.
	* NEWS: Mention the new feature.
        * testsuite/gas/elf/attach-1.s: New test.
        * testsuite/gas/elf/attach-1.d: New test driver.
        * testsuite/gas/elf/attach-2.s: New test.
        * testsuite/gas/elf/attach-2.d: New test driver.
        * testsuite/gas/elf/attach-err.s: New test.
        * testsuite/gas/elf/attach-err.d: New test driver.
        * testsuite/gas/elf/attach-err.err: New test error output.
        * testsuite/gas/elf/elf.exp: Run the new tests.