binfmt_elf_fdpic: fix /proc/<pid>/auxv

binfmt_elf_fdpic: fix /proc/<pid>/auxv

Althought FDPIC linux kernel provides /proc/<pid>/auxv files they are
empty because there's no code that initializes mm->saved_auxv in the
FDPIC ELF loader.

Synchronize FDPIC ELF aux vector setup with ELF. Replace entry-by-entry
aux vector copying to userspace with initialization of mm->saved_auxv
first and then copying it to userspace as a whole.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Link: https://lore.kernel.org/r/20240322195418.2160164-1-jcmvbkbc@gmail.com
Signed-off-by: Kees Cook <keescook@chromium.org>