Add support for different relocation models

Rust currently defaults to `RelocPIC` regardless. This patch adds a new
codegen option that allows choosing different relocation-model. The
available models are:

    - default (Use the target-specific default model)
    - static
    - pic
    - no-pic

For a more detailed information use `llc --help`
auto merge of #13340 : FlaPer87/rust/code-model, r=cmr

Rust currently defaults to `RelocPIC` regardless. This patch adds a new
codegen option that allows choosing different relocation-model. The
available models are:

    - default (Use the target-specific default model)
    - static
    - pic
    - no-pic

For a more detailed information use `llc --help`