gas: Generate a new section for SHF_GNU_RETAIN

gas: Generate a new section for SHF_GNU_RETAIN

For
	.globl	foo2
	.section	.data.foo,"aR"
	.align 4
	.type	foo2, @object
	.size	foo2, 4
foo2:
	.long	2
	.globl	foo1
	.section	.data.foo
	.align 4
	.type	foo1, @object
	.size	foo1, 4
foo1:
	.long	1

generate a new section if the SHF_GNU_RETAIN bit doesn't match.

	* config/obj-elf.c (SEC_ASSEMBLER_SHF_MASK): New.
	(get_section_by_match): Also check if SEC_ASSEMBLER_SHF_MASK of
	sh_flags matches.  Rename info to sh_info.
	(obj_elf_change_section): Don't check previous SHF_GNU_RETAIN.
	Rename info to sh_info.
	(obj_elf_section): Rename info to sh_info.  Set sh_flags for
	SHF_GNU_RETAIN.
	* config/obj-elf.h (elf_section_match): Rename info to sh_info.
	Add sh_flags.
	* testsuite/gas/elf/elf.exp: Run section27.
	* testsuite/gas/elf/section24b.d: Updated.
	* testsuite/gas/elf/section27.d: New file.
	* testsuite/gas/elf/section27.s: Likewise.