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, 9 insertions, 5 deletions

diff --git a/library/std/src/sys/fd/mod.rs b/library/std/src/sys/fd/mod.rs
index e0f5eab6951..7cb9dd1cba9 100644
--- a/library/std/src/sys/fd/mod.rs
+++ b/library/std/src/sys/fd/mod.rs
@@ -2,18 +2,22 @@
 
 #![forbid(unsafe_op_in_unsafe_fn)]
 
-cfg_if::cfg_if! {
-    if #[cfg(target_family = "unix")] {
+cfg_select! {
+    target_family = "unix" => {
         mod unix;
         pub use unix::*;
-    } else if #[cfg(target_os = "hermit")] {
+    }
+    target_os = "hermit" => {
         mod hermit;
         pub use hermit::*;
-    } else if #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] {
+    }
+    all(target_vendor = "fortanix", target_env = "sgx") => {
         mod sgx;
         pub use sgx::*;
-    } else if #[cfg(target_os = "wasi")] {
+    }
+    target_os = "wasi" => {
         mod wasi;
         pub use wasi::*;
     }
+    _ => {}
 }