[6.3] Wasm: bump default stack size to 128K (#2030)

Cherry-pick of #2026.

**Explanation**: The default of 64K for stack size is too low, especially as tail calls are not available by default on Wasm. It also means Swift Testing is unusable in `main` development snapshots due to stack overflows.

**Scope**: Limited to Wasm support.
**Risk**: Low due to isolated scope.
**Testing**: Added automated end-to-end test in swiftlang/swift-integration-tests#167.
**Issue**: rdar://160218251
**Reviewer**: @kateinoigakukun

Author: MaxDesiatov

Sources/SwiftDriver/Jobs/WebAssemblyToolchain+LinkerSupport.swift

@@ -166,6 +166,14 @@ extension WebAssemblyToolchain {
       commandLine.appendFlag(.Xlinker)
       commandLine.appendFlag("--table-base=\(SWIFT_ABI_WASM32_LEAST_VALID_POINTER)")
 
+      // Set slightly higher than the default (64K) stack size so that basic
+      // workflows like running Swift Testing can proceed within this limited stack space.
+      let SWIFT_WASM_DEFAULT_STACK_SIZE = 1024 * 128
+      commandLine.appendFlag(.Xlinker)
+      commandLine.appendFlag("-z")
+      commandLine.appendFlag(.Xlinker)
+      commandLine.appendFlag("stack-size=\(SWIFT_WASM_DEFAULT_STACK_SIZE)")
+
       // Delegate to Clang for sanitizers. It will figure out the correct linker
       // options.
       if linkerOutputType == .executable && !sanitizers.isEmpty {

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -2766,6 +2766,7 @@ final class SwiftDriverTests: XCTestCase {
         XCTAssertTrue(cmd.contains(.responseFilePath(.absolute(path.appending(components: "wasi", "static-executable-args.lnk")))))
         XCTAssertTrue(cmd.contains(subsequence: [.flag("-Xlinker"), .flag("--global-base=4096")]))
         XCTAssertTrue(cmd.contains(subsequence: [.flag("-Xlinker"), .flag("--table-base=4096")]))
+        XCTAssertTrue(cmd.contains(subsequence: [.flag("-Xlinker"), .flag("-z"), .flag("-Xlinker"), .flag("stack-size=\(128 * 1024)")]))
         XCTAssertTrue(cmd.contains(.flag("-O3")))
         XCTAssertEqual(linkJob.outputs[0].file, try toPath("Test"))