Rollup merge of #122003 - mati865:gnullvm-build-libunwind, r=petrochenkov

link libunwind dynamically and allow controlling it via `crt-static` on gnullvm targets

Alternative to https://github.com/rust-lang/rust/pull/121794

```
$ cargo b -r
    Finished `release` profile [optimized] target(s) in 0.38s

$ ntldd target/release/hello.exe | rg unwind
        libunwind.dll => H:\msys64\clang64\bin\libunwind.dll (0x0000020c35df0000)

$ RUSTFLAGS="-C target-feature=+crt-static" cargo b -r
    Finished `release` profile [optimized] target(s) in 0.23s

$ ntldd target/release/hello.exe | rg unwind
```

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

@@ -42,6 +42,8 @@ pub(crate) fn opts() -> TargetOptions {
         eh_frame_header: false,
         no_default_libraries: false,
         has_thread_local: true,
+        crt_static_allows_dylibs: true,
+        crt_static_respected: true,
         // FIXME(davidtwco): Support Split DWARF on Windows GNU - may require LLVM changes to
         // output DWO, despite using DWARF, doesn't use ELF..
         debuginfo_kind: DebuginfoKind::Pdb,