Auto merge of #100341 - andrewpollack:fuchsia-llvm-libunwind, r=tmandry

Use llvm-libunwind="in-tree" for Fuchsia targets

With updates to Fuchsia CI's Zircon libraries #99833, we can introduce `llvm-libunwind="in-tree"` for Fuchsia targets. This PR restores functionality removed from #93604 (comment).

cc `@tmandry` `@djkoloski`

Author: bors

Diff for: src/bootstrap/config.rs

@@ -1445,7 +1445,11 @@ impl Config {
             .get(&target)
             .and_then(|t| t.llvm_libunwind)
             .or(self.llvm_libunwind_default)
-            .unwrap_or(LlvmLibunwind::No)
+            .unwrap_or(if target.contains("fuchsia") {
+                LlvmLibunwind::InTree
+            } else {
+                LlvmLibunwind::No
+            })
     }
 
     pub fn submodules(&self, rust_info: &GitInfo) -> bool {

Diff for: src/doc/rustc/src/platform-support/fuchsia.md

@@ -53,12 +53,6 @@ In `config.toml`, add:
 ```toml
 [build]
 target = ["<host_platform>", "aarch64-fuchsia", "x86_64-fuchsia"]
-
-[target.x86_64-fuchsia]
-llvm-libunwind = "in-tree"
-
-[target.aarch64-fuchsia]
-llvm-libunwind = "in-tree"
 ```
 
 Additionally, the following environment variables must be configured (for