Add a small helper, sym_add_unresolved() to ease the further
refactoring.

Remove the 'weak' argument from alloc_symbol() because it is sensible
only for unresolved symbols.
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>
Reviewed-by: NNick Desaulniers <ndesaulniers@google.com>