* Update how wasi-sdk uses cmake
Change `CMAKE_SYSTEM_NAME` to `WASI`, use a `CMAKE_MODULE_PATH`,
remove `CACHE` usage, and fix the compiler autodetection code so that
the "compiler works" hacks are no longer needed, following the advice in
[this comment](https://gitlab.kitware.com/cmake/cmake/-/issues/19223#note_567327).
This is hopefully a step towards re-submitting WASI support to upstream cmake.
* Add --sysroot to compiler-rt's C flags.
This ensures that it finds the built sysroot when testing whether the C
compiler works.
* Fixes.
* Use the just-built ar rather than the host ar.
* Use -DCMAKE_C_COMPILER_WORKS=ON instead of stubbing out archive files.
* Use -DCMAKE_CXX_COMPILER_WORKS=ON too.
* Restore the `--sysroot` argument when building compiler-rt.
compiler-rt depends on some libc headers, so it needs a sysroot.
* Add sysroot arguments for libc++ and libc++abi too.
* Also set CMAKE_AR.
It appears with the removal of the AR checks cmake is no longer
autodetecting CMAKE_AR, so set it explicitly.
Dan Gohman