This new flag makes it easy to build Swift cross-compilation toolchains,
by disabling cross-compilation of all host tools, like the Swift
compiler and various macros, building on prior pulls #38441 and #82163.
Native compilation has more fine-grained flags like
`--build-swift-tools`, `--build-llvm`, and `--swift-testing-macros`, but
those will disable building them for _all_ platforms (with the exception
of `--build-swift-tools`, which I modified to only apply for the host,
since the initial introduction of this
`--cross-compile-build-swift-tools` flag a couple months ago), so
they're not suited for building a cross-compilation toolchain, where you
want all host tools built for the native host but not for any
cross-compilation SDKs.
It makes sense to just set this here since we aren't testing that functionality
and enables us to test this if we aren't using one of the specified Xcodes
(which can be useful).
Pass the Swift and CMake flags needed to cross-compile Foundation and so on, with
the first example of Android. Add a new flag, --cross-compile-deps-path, which is
used to search for cross-compiled libraries, like libcurl, that the corelibs
depend on. Also add a new flag, --common-swift-flags, to pass additional Swift
flags to the corelibs.
