add `rustflags` option to `bootstrap.toml`

[karolzwolak]

Add a rustflags like you can do in cargo configuration.
This option should just append it's contents at the end of the end of rustflags passed to the compiler.

This consists of implementing:

• global rust.rustflags (feat: add rust.rustflags option to bootstrap.toml #148795)
• maybe add rustflags-bootstrap and rustflags-no-bootstrap like it's done to env variables?
• maybe rustflags per target ?