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.

3 files changed, 26 insertions, 16 deletions

diff --git a/library/std_detect/src/detect/os/linux/auxvec.rs b/library/std_detect/src/detect/os/linux/auxvec.rs
index 443caaaa186..75e01bdc449 100644
--- a/library/std_detect/src/detect/os/linux/auxvec.rs
+++ b/library/std_detect/src/detect/os/linux/auxvec.rs
@@ -131,15 +131,15 @@ pub(crate) fn auxv() -> Result<AuxVec, ()> {
 /// `getauxval` function. If the function is not linked, this function return `Err`.
 fn getauxval(key: usize) -> Result<usize, ()> {
     type F = unsafe extern "C" fn(libc::c_ulong) -> libc::c_ulong;
-    cfg_if::cfg_if! {
-        if #[cfg(all(
+    cfg_select! {
+        all(
             feature = "std_detect_dlsym_getauxval",
             not(all(
                 target_os = "linux",
                 any(target_env = "gnu", target_env = "musl", target_env = "ohos"),
             )),
             not(target_os = "android"),
-        ))] {
+        ) => {
             let ffi_getauxval: F = unsafe {
                 let ptr = libc::dlsym(libc::RTLD_DEFAULT, c"getauxval".as_ptr());
                 if ptr.is_null() {
@@ -147,7 +147,8 @@ fn getauxval(key: usize) -> Result<usize, ()> {
                 }
                 core::mem::transmute(ptr)
             };
-        } else {
+        }
+        _ => {
             let ffi_getauxval: F = libc::getauxval;
         }
     }
diff --git a/library/std_detect/src/detect/os/linux/auxvec/tests.rs b/library/std_detect/src/detect/os/linux/auxvec/tests.rs
index 536615fa272..631a3e5e9ef 100644
--- a/library/std_detect/src/detect/os/linux/auxvec/tests.rs
+++ b/library/std_detect/src/detect/os/linux/auxvec/tests.rs
@@ -42,8 +42,8 @@ fn auxv_dump() {
 }
 
 #[cfg(feature = "std_detect_file_io")]
-cfg_if::cfg_if! {
-    if #[cfg(target_arch = "arm")] {
+cfg_select! {
+    target_arch = "arm" => {
         // The tests below can be executed under qemu, where we do not have access to the test
         // files on disk, so we need to embed them with `include_bytes!`.
         #[test]
@@ -62,7 +62,8 @@ cfg_if::cfg_if! {
             assert_eq!(v.hwcap, 126614527);
             assert_eq!(v.hwcap2, 0);
         }
-    } else if #[cfg(target_arch = "aarch64")] {
+    }
+    target_arch = "aarch64" => {
         #[cfg(target_endian = "little")]
         #[test]
         fn linux_artificial_aarch64() {
@@ -81,6 +82,7 @@ cfg_if::cfg_if! {
             assert_eq!(v.hwcap2, 0);
         }
     }
+    _ => {}
 }
 
 #[test]
diff --git a/library/std_detect/src/detect/os/linux/mod.rs b/library/std_detect/src/detect/os/linux/mod.rs
index 5ae2aaeab5b..5273c16c089 100644
--- a/library/std_detect/src/detect/os/linux/mod.rs
+++ b/library/std_detect/src/detect/os/linux/mod.rs
@@ -37,29 +37,36 @@ fn read_file(orig_path: &str) -> Result<Vec<u8>, alloc::string::String> {
     }
 }
 
-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(any(target_arch = "riscv32", target_arch = "riscv64"))] {
+    }
+    any(target_arch = "riscv32", target_arch = "riscv64") => {
         mod riscv;
         pub(crate) use self::riscv::detect_features;
-    } else if #[cfg(any(target_arch = "mips", target_arch = "mips64"))] {
+    }
+    any(target_arch = "mips", target_arch = "mips64") => {
         mod mips;
         pub(crate) use self::mips::detect_features;
-    } else if #[cfg(any(target_arch = "powerpc", target_arch = "powerpc64"))] {
+    }
+    any(target_arch = "powerpc", target_arch = "powerpc64") => {
         mod powerpc;
         pub(crate) use self::powerpc::detect_features;
-    } else if #[cfg(any(target_arch = "loongarch32", target_arch = "loongarch64"))] {
+    }
+    any(target_arch = "loongarch32", target_arch = "loongarch64") => {
         mod loongarch;
         pub(crate) use self::loongarch::detect_features;
-    } else if #[cfg(target_arch = "s390x")] {
+    }
+    target_arch = "s390x" => {
         mod s390x;
         pub(crate) use self::s390x::detect_features;
-    } else {
+    }
+    _ => {
         use crate::detect::cache;
         /// Performs run-time feature detection.
         pub(crate) fn detect_features() -> cache::Initializer {