[llvm] gn build: Support llvm_enable_zstd. (PR #88457)

[Arthur Eubanks via llvm-commits]

================
@@ -207,6 +207,9 @@ template("stage2_unix_toolchain") {
       ar = "bin/llvm-ar"
       deps += [ "//:llvm-ar($host_toolchain)" ]
     }
+    if (toolchain_args.current_os != host_os) {
----------------
aeubanks wrote:

should we just unconditionally set `llvm_enable_zstd = false` above in `toolchain_args`? and it'd be nice for `llvm_enable_zlib` to be consistent

https://github.com/llvm/llvm-project/pull/88457