Enable --gc-sections for WebAssembly targets (#7356)

Enable `--gc-sections` back for WebAssembly targets

### Motivation:

We disabled `--gc-sections` for Wasm targets due to a lack of features
in the object file format. However, we recently added the missing piece
in wasm object file format, so we no longer have any reason to disable
it.

### Modifications:

This effectively reverts 3a366cc. After
swiftlang/swift#71768, we don't have any reason to
disable `--gc-sections` for WebAssembly targets. Note that the new wasm
segment flags are only supported by the latest LLVM and wasm-ld, but we
can assume that toolchains or Swift SDKs for WebAssembly have wasm-ld
built from the latest LLVM.


### Result:

Allow dead code stripping at link time for WebAssembly targets.

Author: kateinoigakukun

Sources/Build/BuildDescription/ProductBuildDescription.swift

@@ -122,15 +122,6 @@ public final class ProductBuildDescription: SPMBuildCore.ProductBuildDescription
                 return ["-Xlinker", "-dead_strip"]
             } else if triple.isWindows() {
                 return ["-Xlinker", "/OPT:REF"]
-            } else if triple.arch == .wasm32 {
-                // FIXME: wasm-ld strips data segments referenced through __start/__stop symbols
-                // during GC, and it removes Swift metadata sections like swift5_protocols
-                // We should add support of SHF_GNU_RETAIN-like flag for __attribute__((retain))
-                // to LLVM and wasm-ld
-                // This workaround is required for not only WASI but also all WebAssembly triples
-                // using wasm-ld (e.g. wasm32-unknown-unknown). So this branch is conditioned by
-                // arch == .wasm32
-                return []
             } else {
                 return ["-Xlinker", "--gc-sections"]
             }

Tests/BuildTests/CrossCompilationBuildPlanTests.swift

@@ -70,6 +70,39 @@ final class CrossCompilationBuildPlanTests: XCTestCase {
         )
     }
 
+    func testWasmTargetRelease() throws {
+        let pkgPath = AbsolutePath("/Pkg")
+
+        let (graph, fs, observabilityScope) = try trivialPackageGraph(pkgRootPath: pkgPath)
+
+        var parameters = mockBuildParameters(
+            config: .release, targetTriple: .wasi, linkerDeadStrip: true
+        )
+        parameters.linkingParameters.shouldLinkStaticSwiftStdlib = true
+        let result = try BuildPlanResult(plan: BuildPlan(
+            buildParameters: parameters,
+            graph: graph,
+            fileSystem: fs,
+            observabilityScope: observabilityScope
+        ))
+        let buildPath = result.plan.productsBuildPath
+
+        let appBuildDescription = try result.buildProduct(for: "app")
+        XCTAssertEqual(
+            try appBuildDescription.linkArguments(),
+            [
+                result.plan.destinationBuildParameters.toolchain.swiftCompilerPath.pathString,
+                "-L", buildPath.pathString,
+                "-o", buildPath.appending(components: "app.wasm").pathString,
+                "-module-name", "app", "-static-stdlib", "-emit-executable",
+                "-Xlinker", "--gc-sections",
+                "@\(buildPath.appending(components: "app.product", "Objects.LinkFileList"))",
+                "-target", "wasm32-unknown-wasi",
+                "-g",
+            ]
+        )
+    }
+
     func testWASITarget() throws {
         let pkgPath = AbsolutePath("/Pkg")