Rollup merge of rust-lang#45739 - rkarp:master, r=petrochenkov

Fix libstd compile error for windows-gnu targets without `backtrace`

This is basically an addition to rust-lang#44979. Compiling `libstd` still fails when targeting `windows-gnu` with `panic = "abort"` because the items in the `...c::gnu` module are not used. They are only referenced from `backtrace_gnu.rs`, which is indirectly feature gated behind `backtrace` [here](https://github.com/rust-lang/rust/blob/9f3b09116b742b2606dc5f36f9145e0c89e4010b/src/libstd/sys/windows/mod.rs#L23).

Author: kennytm

src/libstd/sys/windows/c.rs

@@ -1228,7 +1228,7 @@ compat_fn! {
     }
 }
 
-#[cfg(target_env = "gnu")]
+#[cfg(all(target_env = "gnu", feature = "backtrace"))]
 mod gnu {
     use super::*;
 
@@ -1256,5 +1256,5 @@ mod gnu {
     }
 }
 
-#[cfg(target_env = "gnu")]
+#[cfg(all(target_env = "gnu", feature = "backtrace"))]
 pub use self::gnu::*;