Conditionally build wasm-component-ld

This commit updates the support for the `wasm-component-ld` tool
from #126967 to conditionally build it rather than unconditionally
building it when LLD is enabled. This support is disabled by default and
can be enabled by one of two means:

* the `extended` field in `config.toml` which dist builders use to build
  a complete set of tools for each host platform.
* a `"wasm-component-ld"` entry in the `tools` section of `config.toml`.

Neither of these are enabled by default meaning that most local builds
will likely not have this new tool built. Dist builders should still,
however, build the tool.

config.example.toml

@@ -333,6 +333,7 @@
 #    "rust-analyzer-proc-macro-srv",
 #    "analysis",
 #    "src",
+#    "wasm-component-ld",
 #]
 
 # Verbosity level: 0 == not verbose, 1 == verbose, 2 == very verbose, 3 == print environment variables on each rustc invocation

src/bootstrap/src/core/build_steps/compile.rs

@@ -1820,12 +1820,14 @@ fn run(self, builder: &Builder<'_>) -> Compiler {
                     &self_contained_lld_dir.join(exe(name, target_compiler.host)),
                 );
             }
+        }
 
         // In addition to `rust-lld` also install `wasm-component-ld` when
         // LLD is enabled. This is a relatively small binary that primarily
         // delegates to the `rust-lld` binary for linking and then runs
         // logic to create the final binary. This is used by the
         // `wasm32-wasip2` target of Rust.
+        if builder.build_wasm_component_ld() {
             let wasm_component_ld_exe =
                 builder.ensure(crate::core::build_steps::tool::WasmComponentLd {
                     compiler: build_compiler,

src/bootstrap/src/lib.rs

@@ -1414,6 +1414,19 @@ fn default_wasi_runner(&self) -> Option<String> {
         None
     }
 
+    /// Returns whether it's requested that `wasm-component-ld` is built as part
+    /// of the sysroot. This is done either with the `extended` key in
+    /// `config.toml` or with the `tools` set.
+    fn build_wasm_component_ld(&self) -> bool {
+        if self.config.extended {
+            return true;
+        }
+        match &self.config.tools {
+            Some(set) => set.contains("wasm-component-ld"),
+            None => false,
+        }
+    }
+
     /// Returns the root of the "rootfs" image that this target will be using,
     /// if one was configured.
     ///