binfmt_elf: Use elf_load() for library

While load_elf_library() is a libc5-ism, we can still replace most of
its contents with elf_load() as well, further simplifying the code.

Some historical context:
- libc4 was a.out and used uselib (a.out support has been removed)
- libc5 was ELF and used uselib (there may still be users)
- libc6 is ELF and has never used uselib

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mm@kvack.org
Suggested-by: Eric Biederman <ebiederm@xmission.com>
Tested-by: Pedro Falcato <pedro.falcato@gmail.com>
Signed-off-by: Sebastian Ott <sebott@redhat.com>
Link: https://lore.kernel.org/r/20230929032435.2391507-4-keescook@chromium.org


Signed-off-by: Kees Cook <keescook@chromium.org>