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, 16 insertions, 10 deletions

diff --git a/library/std/src/sys/sync/thread_parking/mod.rs b/library/std/src/sys/sync/thread_parking/mod.rs
index f4d8fa0a58c..b9fb27b4eef 100644
--- a/library/std/src/sys/sync/thread_parking/mod.rs
+++ b/library/std/src/sys/sync/thread_parking/mod.rs
@@ -1,5 +1,5 @@
-cfg_if::cfg_if! {
-    if #[cfg(any(
+cfg_select! {
+    any(
         all(target_os = "windows", not(target_vendor = "win7")),
         target_os = "linux",
         target_os = "android",
@@ -9,30 +9,36 @@ cfg_if::cfg_if! {
         target_os = "dragonfly",
         target_os = "fuchsia",
         target_os = "hermit",
-    ))] {
+    ) => {
         mod futex;
         pub use futex::Parker;
-    } else if #[cfg(any(
+    }
+    any(
         target_os = "netbsd",
         all(target_vendor = "fortanix", target_env = "sgx"),
         target_os = "solid_asp3",
-    ))] {
+    ) => {
         mod id;
         pub use id::Parker;
-    } else if #[cfg(target_vendor = "win7")] {
+    }
+    target_vendor = "win7" => {
         mod windows7;
         pub use windows7::Parker;
-    } else if #[cfg(all(target_vendor = "apple", not(miri)))] {
+    }
+    all(target_vendor = "apple", not(miri)) => {
         // Doesn't work in Miri, see <https://github.com/rust-lang/miri/issues/2589>.
         mod darwin;
         pub use darwin::Parker;
-    } else if #[cfg(target_os = "xous")] {
+    }
+    target_os = "xous" => {
         mod xous;
         pub use xous::Parker;
-    } else if #[cfg(target_family = "unix")] {
+    }
+    target_family = "unix" => {
         mod pthread;
         pub use pthread::Parker;
-    } else {
+    }
+    _ => {
         mod unsupported;
         pub use unsupported::Parker;
     }