kernel: add support for  .init_array.* constructors

Note: We don't use SORT_BY_INIT_PRIORITY() here to avoid
randconfig build failures with old binutils which may not
support it. In this use case SORT() will give us the same
result as SORT_BY_INIT_PRIORITY(), so we use it.

Signed-off-by: Andrey Ryabinin <a.ryabinin@samsung.com>