elf: Add initial flag argument to __libc_early_init

The rseq initialization should happen only for the libc in the base
namespace (in the dynamic case) or the statically linked libc.  The
__libc_multiple_libcs flag does not quite cover this case at present,
so this commit introduces a flag argument to __libc_early_init,
indicating whether the libc being libc is the primary one (of the main
program).

Reviewed-by: Carlos O'Donell <carlos@redhat.com>