Add CMake for Emscripten-hosted compiler (#90334)
Added three cache files: one each for the cross-LLVM and the Swift
compiler, the third is a top-level include that pre-defines the lld
imported targets `find_package(Clang)` expects. Two existing CMake files
gain `SWIFT_HOST_VARIANT_SDK == EMSCRIPTEN` branches for the runtime
link and module include paths, which don't affect other hosts. The one
non-gated change wraps a `CROSSCOMPILE` swift-stdlib ordering dependency
in an existence check. It is a no-op unless that target is absent, which
happens only when building against a prebuilt stdlib.
