library: Migrate from `cfg_if` to `cfg_select`

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.

1 files changed, 18 insertions, 11 deletions

diff --git a/library/std/src/sys/io/mod.rs b/library/std/src/sys/io/mod.rs
index ae75f4d97b4..fe8ec1dbb73 100644
--- a/library/std/src/sys/io/mod.rs
+++ b/library/std/src/sys/io/mod.rs
@@ -1,20 +1,24 @@
 #![forbid(unsafe_op_in_unsafe_fn)]
 
 mod io_slice {
-    cfg_if::cfg_if! {
-        if #[cfg(any(target_family = "unix", target_os = "hermit", target_os = "solid_asp3", target_os = "trusty"))] {
+    cfg_select! {
+        any(target_family = "unix", target_os = "hermit", target_os = "solid_asp3", target_os = "trusty") => {
             mod iovec;
             pub use iovec::*;
-        } else if #[cfg(target_os = "windows")] {
+        }
+        target_os = "windows" => {
             mod windows;
             pub use windows::*;
-        } else if #[cfg(target_os = "wasi")] {
+        }
+        target_os = "wasi" => {
             mod wasi;
             pub use wasi::*;
-        } else if #[cfg(target_os = "uefi")] {
+        }
+        target_os = "uefi" => {
             mod uefi;
             pub use uefi::*;
-        } else {
+        }
+        _ => {
             mod unsupported;
             pub use unsupported::*;
         }
@@ -22,17 +26,20 @@ mod io_slice {
 }
 
 mod is_terminal {
-    cfg_if::cfg_if! {
-        if #[cfg(any(target_family = "unix", target_os = "wasi"))] {
+    cfg_select! {
+        any(target_family = "unix", target_os = "wasi") => {
             mod isatty;
             pub use isatty::*;
-        } else if #[cfg(target_os = "windows")] {
+        }
+        target_os = "windows" => {
             mod windows;
             pub use windows::*;
-        } else if #[cfg(target_os = "hermit")] {
+        }
+        target_os = "hermit" => {
             mod hermit;
             pub use hermit::*;
-        } else {
+        }
+        _ => {
             mod unsupported;
             pub use unsupported::*;
         }