https://github.com/rust-lang/rust/pull/145489 changed `std_detect` to no
longer depend on `cfg-if`, which meant it no longer indirectly pulled in
`rustc-std-workspace-core` via `cfg-if`. That caused it to no longer
depend on `compiler-builtins`.
Change `std_detect` to use `rustc-std-workspace-core` and
`rustc-std-workspace-alloc`, to integrate with the rustc workspace. This
also pulls in `compiler-builtins` via `rustc-std-workspace-core`.
Closes: https://github.com/rust-lang/rust/issues/145594
Migrate the standard library from using the external `cfg_if` crate to
using the now-built-in `cfg_select` macro.
This does not yet eliminate the dependency from
`library/std/Cargo.toml`, because while the standard library itself no
longer uses `cfg_if`, it also incorporates the `backtrace` crate, which
does.
Migration assisted by the following vim command (after selecting the
full `cfg_if!` invocation):
```
'<,'>s/\(cfg_if::\)\?cfg_if/cfg_select/ | '<,'>s/^\( *\)} else {/\1}\r\1_ => {/c | '<,'>s/^\( *\)} else if #\[cfg(\(.*\))\] /\1}\r\1\2 => /e | '<,'>s/if #\[cfg(\(.*\))\] {/\1 => {/e
```
This is imperfect, but substantially accelerated the process. This
prompts for confirmation on the `} else {` since that can also appear
inside one of the arms. This also requires manual intervention to handle
any multi-line conditions.
