Support specifying target architecture

[davoclavo]

Currently ruskel seems to assume that the crate is compiled for the same architecture.

Example running on latest ruskel (57ad247)

cargo run --bin ruskel -- --verbose esp-hal --features esp32c6

error[E0432]: unresolved import `esp_riscv_rt`
   --> src/lib.rs:149:24
    |
149 | pub use esp_riscv_rt::{self, riscv};
    |                        ^^^^ no external crate `esp_riscv_rt`

error[E0432]: unresolved import `esp_riscv_rt`
   --> src/lib.rs:296:13
    |
296 |     pub use esp_riscv_rt::TrapFrame;
    |             ^^^^^^^^^^^^ use of unresolved module or unlinked crate `esp_riscv_rt`
    |
    = help: if you wanted to use a crate named `esp_riscv_rt`, use `cargo add esp_riscv_rt` to add it to your `Cargo.toml`

...

This one fails because it is meant to be compiled for a variant of riscv architecture: riscv32imac-unknown-none-elf

Btw, kind of unrelated but just noticed that the real error message gets swallowed if we omit the --verbose flag. Otherwise it only shows this error:

ruskel requires the nightly toolchain to be installed - run 'rustup toolchain install nightly'

(which is a bit confusing as the nightly toolchain already installed and in the latest version)