Ignore code model setting for m68k

[techmetx11]

m68k-unknown-none-elf's target spec has the code model set to medium which causes rustc_codegen_gcc to insert -mcmodel=medium in the GCC command-line options. Since GCC m68k doesn't support this option, it prematurely errors with libgccjit.so: error: unrecognized command-line option '-mcmodel=medium'

This PR fixes the issue by making rustc_codegen_gcc ignore the code model setting if the target architecture is m68k

[antoyo]

Would it make more sense to fix the issue upstream in the Rust repo?
(i.e. can cg_llvm set the code model on that target?)
Perhaps by removing the code model from this target spec?

[techmetx11]

Would it make more sense to fix the issue upstream in the Rust repo? (i.e. can cg_llvm set the code model on that target?) Perhaps by removing the code model from this target spec?

Probably would, since the m68k-unknown-linux-gnu target spec has no code model set. I don't know why the code model is set specifically for this target