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, 8 insertions, 5 deletions

diff --git a/library/std_detect/src/detect/os/freebsd/mod.rs b/library/std_detect/src/detect/os/freebsd/mod.rs
index ade7fb6269d..7de9250e835 100644
--- a/library/std_detect/src/detect/os/freebsd/mod.rs
+++ b/library/std_detect/src/detect/os/freebsd/mod.rs
@@ -2,17 +2,20 @@
 
 mod auxvec;
 
-cfg_if::cfg_if! {
-    if #[cfg(target_arch = "aarch64")] {
+cfg_select! {
+    target_arch = "aarch64" => {
         mod aarch64;
         pub(crate) use self::aarch64::detect_features;
-    } else if #[cfg(target_arch = "arm")] {
+    }
+    target_arch = "arm" => {
         mod arm;
         pub(crate) use self::arm::detect_features;
-    } else if #[cfg(target_arch = "powerpc64")] {
+    }
+    target_arch = "powerpc64" => {
         mod powerpc;
         pub(crate) use self::powerpc::detect_features;
-    } else {
+    }
+    _ => {
         use crate::detect::cache;
         /// Performs run-time feature detection.
         pub(crate) fn detect_features() -> cache::Initializer {