Don't build rustc without std
Don't build rustc without std

- Set rustc to build only when explicitly asked for

This allows building the stage2 rustc artifacts, which nothing depends
on.

Previously the behavior was as follows (where stageN <-> stage(N-1) artifacts, except for stage0 libstd):

- `x.py build --stage 0`:
  - stage0 libstd
  - stage1 rustc (but without putting rustc in stage0/)

This leaves you without any rustc at all except for the beta compiler
(https://github.com/rust-lang/rust/issues/73519). This is never what you want.

- `x.py build --stage 1`:
  - stage0 libstd
  - stage1 rustc
  - stage1 libstd
  - stage1 rustdoc
  - stage2 rustc

This leaves you with a broken stage2 rustc which doesn't even have
libcore and is effectively useless. Additionally, it compiles rustc
twice, which is not normally what you want.

- `x.py build --stage 2`:
  - stage0 libstd
  - stage1 rustc
  - stage1 libstd
  - stage2 rustc
  - stage2 rustdoc and tools

This builds all tools in release mode. This is the correct usage for CI,
but takes far to long for development.

Now the behavior is as follows:

- `x.py build --stage 0`:
  - stage0 libstd

This is suitable for contributors only working on the standard library,
as it means rustc never has to be compiled.

- `x.py build --stage 1`:
  - stage0 libstd
  - stage1 rustc
  - stage1 libstd
  - stage1 rustdoc

This is suitable for contributors working on the compiler. It ensures
that you have a working rustc and libstd without having to pass
`src/libstd` in addition.

- `x.py build --stage 2`:
  - stage0 libstd
  - stage1 rustc
  - stage1 libstd
  - stage2 rustc
  - stage2 libstd
  - stage2 rustdoc

This is suitable for debugging errors which only appear with the stage2
compiler.

- `x.py build --stage 2 src/libstd src/rustc`
  - stage0 libstd
  - stage1 rustc
  - stage1 libstd
  - stage2 rustc
  - stage2 libstd
  - stage2 rustdoc, tools, etc.
  - stage2 rustc artifacts ('stage3')

This is suitable for CI, which wants all tools in release mode.
However, most of the use cases for this should use `x.py dist` instead,
which builds all the tools without each having to be named individually.

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust