build: Add support for wasi-sysroot option in build-script

`wasi-sysroot` allows providing a path to a custom libc that supports this platform.

Author: MaxDesiatov

utils/build-script-impl

@@ -116,12 +116,16 @@ KNOWN_SETTINGS=(
     darwin-toolchain-require-use-os-runtime       "0"               "When setting up a plist for a toolchain, require the users of the toolchain to link against the OS instead of the packaged toolchain runtime. 0 for false, 1 for true"
     darwin-xcrun-toolchain                        "default"         "the name of the toolchain to use on Darwin"
 
+    ## WebAssembly/WASI Options
+    wasi-sysroot                                  ""                "An absolute path to the wasi-sysroot that will be used as a libc implementation for Wasm builds"
+
     ## Build Types for Components
     swift-stdlib-build-type                       "Debug"           "the CMake build variant for Swift"
 
     ## Skip Build ...
     skip-build                                    ""                "set to configure as usual while skipping the build step"
     skip-build-android                            ""                "set to skip building Swift stdlibs for Android"
+    skip-build-wasm                               ""                "set to skip building Swift stdlibs for WebAssembly"
     skip-build-benchmarks                         ""                "set to skip building Swift Benchmark Suite"
     skip-build-clang-tools-extra                  ""                "set to skip building clang-tools-extra as part of llvm"
     skip-build-compiler-rt                        ""                "set to skip building Compiler-RT"
@@ -1741,6 +1745,13 @@ for host in "${ALL_HOSTS[@]}"; do
                     )
                 fi
 
+                if [[ ! "${SKIP_BUILD_WASM}" ]]; then
+                    cmake_options=(
+                        "${cmake_options[@]}"
+                        -DSWIFT_WASI_SYSROOT_PATH:STRING="${WASI_SYSROOT}"
+                    )
+                fi
+
                 if [[ $(is_cross_tools_host ${host}) && "${host}" == "openbsd-"* && -n "${OPENBSD_USE_TOOLCHAIN_FILE}" ]]; then
                     cmake_options=(
                         "${cmake_options[@]}"