Rollup merge of #112435 - antoyo:allow-overwrite-sysroot, r=Mark-Simulacrum

Allow overwriting the sysroot compile flag via --rustc-args

Hi.
As discussed on [Zulip](https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/How.20to.20overwrite.20sysroot.20in.20x.2Epy.20test/near/364272269), this is a solution to allow the codegens to overwrite the sysroot as part of their test suite.
Thanks for the review.

Author: matthiaskrgr