rustc_llvm: Link zlib on cross Solaris builds

On native builds `llvm-config` picks up `zlib` and this gets pased into
the rust build tools, but on cross builds `llvm-config` is explicitly
ignored as it contains information for the host system and cannot be
trusted to be accurate for the target system.

Both DragonFly and Solaris contain `zlib` in the base system, so this is
both a safe assumption and required for a successful cross build unless
`zlib` support is disabled in LLVM.

This is more or less in the same vein as rust-lang#75713 and rust-lang#75655.

compiler/rustc_llvm/build.rs

@@ -251,7 +251,9 @@ fn main() {
     } else if target.contains("windows-gnu") {
         println!("cargo:rustc-link-lib=shell32");
         println!("cargo:rustc-link-lib=uuid");
-    } else if target.contains("haiku") || target.contains("darwin") || target.contains("dragonfly")
+    } else if target.contains("haiku")
+        || target.contains("darwin")
+        || (is_crossed && (target.contains("dragonfly") || target.contains("solaris")))
     {
         println!("cargo:rustc-link-lib=z");
     } else if target.contains("netbsd") {