m68k-unknown-none-elf: Remove code model and change CPU to base MC68000

[techmetx11]

This PR removes the code model because it causes issues when compiling with rustc_codegen_gcc (the code model option doesn't exist in GCC for m68k), and changes the CPU from M68010 to M68000, since it seems like an arbitrary CPU choice over the first CPU in the Motorola 68000 series that the compiler can compile for

r? @knickish

[rustbot]

These commits modify compiler targets.
(See the Target Tier Policy.)

[rustbot]

Failed to set assignee to knickish: invalid assignee

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[rustbot]

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @knickish (or someone else) some time within the next two weeks.

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[techmetx11]

cc @knickish (sorry)