Adds a new CMake option, defaulting to `ON` so existing builds are
unaffected. When set to `OFF`, `wasi-libc` is built with
`-DBUILD_SHARED=OFF` (its existing `if (NOT BUILD_SHARED)` block then
short-circuits the per-target `set(SHARED ON)` for
`wasm32-wasi`/`wasm32-wasip1`), and the `libcxx`/`libcxxabi`/`libunwind`
invocations no longer use `${pic}` directly for their `*_ENABLE_SHARED`
flags — they go through a new local `shared` variable that AND's
`WASI_SDK_BUILD_SHARED` with `${pic}`.
Motivation: downstream consumers that build wasi-sdk from source as part
of a larger toolchain pipeline (in our case, Firefox's build system)
don't ship or link against shared sysroot artifacts. The current code
path provides no way to opt out, so we've been patching locally. The
existing comment in `wasi-sdk-sysroot.cmake` notes shared libraries
aren't supported with LTO; this doesn't change that default, it just
exposes the same toggle for non-LTO builds too.
A `static` entry is added to the `build-only-sysroot` CI matrix to
exercise the OFF path.
Ryan VanderMeulen
d05f757dc2