modpost: introduce module_alias_printf() helper

The generic ->do_entry() handler is currently limited to returning
a single alias string.

However, this is not flexible enough for several subsystems, which
currently require their own implementations:

 - do_usb_table()
 - do_of_table()
 - do_pnp_device_entry()
 - do_pnp_card_entries()

This commit introduces a helper function so that these special cases can
add multiple MODULE_ALIAS() and then migrate to the generic framework.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>