gas: Allow SHF_GNU_RETAIN on all sections
Since SHF_GNU_RETAIN is allowed on all sections, strip SHF_GNU_RETAIN
when checking incorrect section attributes.
PR gas/27412
* config/obj-elf.c (obj_elf_change_section): Strip SHF_GNU_RETAIN
when checking incorrect section attributes.
* testsuite/gas/elf/elf.exp: Run section28 and section29.
* testsuite/gas/elf/section28.d: New file.
* testsuite/gas/elf/section28.s: Likewise.
* testsuite/gas/elf/section29.d: Likewise.
* testsuite/gas/elf/section29.s: Likewise.