windows-gnullvm: Avoid linking to libunwind statically

Avoid linking against the static variant of libunwind, which is not
always available. Instead, prefer to use the unwind library from the
toolchain, which the linker will automatically include, depending
on what's available, and depending on the crt-static feature.

compiler/rustc_target/src/spec/base/windows_gnullvm.rs

@@ -8,7 +8,7 @@ pub fn opts() -> TargetOptions {
     // but LLVM maintainers rejected it: https://reviews.llvm.org/D51440
     let pre_link_args = TargetOptions::link_args(
         LinkerFlavor::Gnu(Cc::Yes, Lld::No),
-        &["-nolibc", "--unwindlib=none"],
+        &["-nolibc"],
     );
     // Order of `late_link_args*` does not matter with LLD.
     let late_link_args = TargetOptions::link_args(

library/unwind/src/lib.rs

@@ -3,7 +3,6 @@
 #![feature(link_cfg)]
 #![feature(staged_api)]
 #![feature(c_unwind)]
-#![cfg_attr(bootstrap, feature(cfg_target_abi))]
 #![feature(strict_provenance)]
 #![cfg_attr(not(target_env = "msvc"), feature(libc))]
 #![allow(internal_features)]

library/unwind/src/libunwind.rs

@@ -102,12 +102,9 @@ pub type _Unwind_Exception_Cleanup_Fn =
 // rustc_codegen_ssa::src::back::symbol_export, rustc_middle::middle::exported_symbols
 // and RFC 2841
 #[cfg_attr(
-    any(
-        all(
-            feature = "llvm-libunwind",
-            any(target_os = "fuchsia", target_os = "linux", target_os = "xous")
-        ),
-        all(target_os = "windows", target_env = "gnu", target_abi = "llvm")
+    all(
+        feature = "llvm-libunwind",
+        any(target_os = "fuchsia", target_os = "linux", target_os = "xous")
     ),
     link(name = "unwind", kind = "static", modifiers = "-bundle")
 )]