PR ld/12761
* elflink.c (elf_link_add_object_symbols): Process .gnu.warning
sections when building shared libraries.
* lib/ld-lib.exp (run_cc_link_tests): Add an optional 8th
parameter - an expected warning message from the linker.
* ld-elf/shared.exp (build_tests): Expect a warning message when
building the libbarw.so library.
* ld-elf/shared.exp: Run library building tests even when not
building a native toolchain.