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, 12 insertions, 7 deletions

diff --git a/library/std/src/sys/path/mod.rs b/library/std/src/sys/path/mod.rs
index a4ff4338cf5..254683bc83f 100644
--- a/library/std/src/sys/path/mod.rs
+++ b/library/std/src/sys/path/mod.rs
@@ -1,22 +1,27 @@
-cfg_if::cfg_if! {
-    if #[cfg(target_os = "windows")] {
+cfg_select! {
+    target_os = "windows" => {
         mod windows;
         mod windows_prefix;
         pub use windows::*;
-    } 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 = "solid_asp3")] {
+    }
+    target_os = "solid_asp3" => {
         mod unsupported_backslash;
         pub use unsupported_backslash::*;
-    } else if #[cfg(target_os = "uefi")] {
+    }
+    target_os = "uefi" => {
         mod uefi;
         pub use uefi::*;
-    } else if #[cfg(target_os = "cygwin")] {
+    }
+    target_os = "cygwin" => {
         mod cygwin;
         mod windows_prefix;
         pub use cygwin::*;
-    } else {
+    }
+    _ => {
         mod unix;
         pub use unix::*;
     }