score and mmix target_id

[Hans-Peter Nilsson]

On Mon, 16 Dec 2024, Alan Modra wrote:

> These targets currently use GENERIC_ELF_DATA as their target_id, but
> that isn't exactly correct.  While their bfd tdata is generic elf,
> their elf_section_data is extended with extra target data.  Add
> MMIX_ELF_DATA and SCORE_ELF_DATA.

This patch broke 29 ld-mmix tests, the first being
ld-mmix/bpo-1.

brgds, H-P