Give a better error when std is missing for a target

[jyn514]

• Note that not all platforms have std pre-compiled
• Suggest cargo build -Zbuild-std on nightly

After rust-lang/rust#84450, you can end up with a situation like this:

$ rustc +stage1 src/main.rs --target x86_64-unknown-uefi
error[E0463]: can't find crate for `core`
  |
  = note: the `x86_64-unknown-uefi` target may not be installed
  = help: consider downloading the target with `rustup target add x86_64-unknown-uefi`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0463`.

$ rustup target add x86_64-unknown-uefi

error: toolchain 'nightly-x86_64-unknown-linux-gnu' does not contain component 'rust-std' for target 'x86_64-unknown-uefi'

That doesn't give you much info on how to proceed. After this PR, it will be more clear:

$ rustup target add x86_64-unknown-uefi

error: toolchain 'nightly-x86_64-unknown-linux-gnu' does not contain component 'rust-std' for target 'x86_64-unknown-uefi'
note: not all platforms have the standard library pre-compiled: https://doc.rust-lang.org/nightly/rustc/platform-support.html
help: consider using `cargo build -Z build-std` instead

[jyn514]

Unlike rust-lang/rust#84450, this recommends -Z build-std whenever you're using a nightly toolchain, because you can always install cargo with rustup and it seems strange to be using rustup but not cargo.

[rbtcollins]

rustup and no cargo will occur when someone builds a rust toolchain from source and links it in. However I agree that this is the common case suggestion to make, and I think anyone using such a non-downloadable toolchain will be sophisticated enough to figure it out with the pointer this gives them. @kinnison thoughts?