Support -Crelocation-model

[bjorn3]

This is necessary for compiling rust-for-linux with cg_gcc.

[antoyo]

It seems the LLVM codegen uses LLVMRustSetDSOLocal: I'm not exactly sure what is the GCC equivalent.

Adding the GCC flags -mcmodel=kernel -fno-pie when relocation_model is static allow compiling rust-for-linux with cg_gcc. But I'm not sure whether this would be correct.

[bjorn3]

Adding the GCC flags -mcmodel=kernel -fno-pie when relocation_model is static allow compiling rust-for-linux with cg_gcc. But I'm not sure whether this would be correct.

Yeah, that worked for me too. There is a bug somewhere in how kernel module parameters are codegened causing the respective sample to crash during early boot though.

The code model option can be directly translated to -mcmodel=<code model> I think. The relocation model will probably need a match and then pass the appropriate -f argument corresponding to LLVM's relocation model.

[antoyo]

My other change to allow running the kernel modules is to remove all the SIMD flags.

[bjorn3]

I added the -mno-... flags that are passed to GCC when by linux when compiling C code. That worked too with the exception of disabling sse. Even with the soft float abi it complains. See #206 (comment).

[antoyo]

Implemented in #578.