Copy elf_gnu_osabi_retain from input only for relocatable link since
SHF_GNU_RETAIN has no impact on non-relocatable outputs.
bfd/
PR ld/27091
* elflink.c (elf_link_input_bfd): Copy elf_gnu_osabi_retain
from input only for relocatable link.
ld/
PR ld/27091
* testsuite/ld-elf/retain7.s: New file.
* testsuite/ld-elf/retain7a.d: Likewise.
* testsuite/ld-elf/retain7b.d: Likewise.