Rollup merge of #122108 - alexcrichton:target-config-runtool, r=WaffleLapkin

Add `target.*.runner` configuration for targets

This commit adds a `runner` field configuration to `config.toml` for specifying a wrapper executable when executing binaries for a target. This is pulled out of #122036 where a WebAssembly runtime is used, for example, to execute tests for `wasm32-wasip1`.

The name "runner" here is chosen to match Cargo's `CARGO_*_RUNNER` configuration, and to make things a bit more consistent this additionally renames compiletest's `--runtool` argument to `--runner`.

Author: jhpratt