[wasm] Repair build for recent SwiftDriver linkage changes

- Before f34d388, the EarlySwiftDriver used Dispatch/Foundation from
old host toolchain at the initial "build" step, and then re-linked the just
built Dispatch/Foundation at "install" step.
- WasmStdlib product incorrectly set `BOOTSTRAPPING_MODE` to `HOSTTOOLS`
and it makes `SwiftSource.cmake` set `LD_LIBRARY_PATH` to load the
Dispatch/Foundation from the just built toolchain (SWIFT_NATIVE_SWIFT_TOOLS_PATH).

After f34d388, the EarlySwiftDriver always uses the corelibs of
the old host toolchain, so setting `LD_LIBRARY_PATH` leads to loading
incompatible corelibs of the old host toolchain, and the build fails.

This change tells the CMake build system to use CROSSCOMPILE mode.
We already built the toolchain for the current host from the revision
being built for the target, so we don't need to inject the lib library
of the old host toolchain.

Author: kateinoigakukun

utils/swift_build_support/swift_build_support/products/wasmstdlib.py

@@ -61,6 +61,8 @@ def _build(self, host_target, target_triple):
             'SWIFT_NATIVE_SWIFT_TOOLS_PATH:STRING', os.path.join(toolchain_path, 'bin'))
         self.cmake_options.define(
             'SWIFT_NATIVE_LLVM_TOOLS_PATH:STRING', os.path.join(toolchain_path, 'bin'))
+        self.cmake_options.define(
+            'BOOTSTRAPPING_MODE:STRING', 'CROSSCOMPILE')
         self.cmake_options.define(
             'SWIFT_BUILD_RUNTIME_WITH_HOST_COMPILER:BOOL', 'FALSE')
         self.cmake_options.define('SWIFT_WASI_SYSROOT_PATH:STRING',