feat: add rust.rustflags and per target rustflags options to bootstrap.toml

This makes easy to persistently pass any flag to the compiler when
building rustc.
For example you can use a different linker by putting the following in
`bootstrap.toml`:
```toml
[rust]
rustflags = ["-Clinker=clang", "-Clink-arg=--ld-path=wild"]
```