swift-wasm-DEVELOPMENT-SNAPSHOT-2021-02-14-a

Simplify build script (#2733)

* [WASM] Simplify Foundation build script

* [WASM] Remove WASI ICU related code from CMake build system

* Refactoring build-toolchain script

* [WASM] Fix prebuilt libraries path

* [WASM] Always install ubuntu wasi-sdk

* Fix unarchive command