build: Add WasmKit (a Wasm runtime) product

This patch adds a new product, WasmKit, which is a Wasm runtime that is
going to be used to run executable tests targeting Wasm. Note that the
product is not shipped as a part of the toolchain, but is used only for
testing purposes.

utils/build_swift/tests/expected_options.py

@@ -129,6 +129,7 @@ EXPECTED_DEFAULTS = {
     'build_watchos_simulator': False,
     'build_xctest': False,
     'build_wasmstdlib': False,
+    'build_wasmkit': False,
     'cmake_c_launcher': None,
     'cmake_cxx_launcher': None,
     'clang_compiler_version': None,
@@ -547,6 +548,7 @@ EXPECTED_OPTIONS = [
     SetTrueOption('--swiftdocc', dest='build_swiftdocc'),
     SetTrueOption('--build-minimal-stdlib', dest='build_minimalstdlib'),
     SetTrueOption('--build-wasm-stdlib', dest='build_wasmstdlib'),
+    SetTrueOption('--wasmkit', dest='build_wasmkit'),
     SetTrueOption('-B', dest='benchmark'),
     SetTrueOption('-S', dest='skip_build'),
     SetTrueOption('-b', dest='build_llbuild'),