This reverts commit 0441a81.

The existing swift-cmake-options flag overwrites all flags computed by
build-script. Sometimes it is useful to be able to append additional
CMake flags without overwriting the existing flags.
This patch adds `--extra-swift-cmake-options` that adds the specified
flags to the Swift CMake configuration instead of overwriting them.

This also adds a similar `--extra-llvm-cmake-options`, which adds the
new flags to the end, allowing one to replace and overwrite CMake flags
that build-script computed.
Due to the parameter passing mechanisms in build-script-impl, while this
behavior would be useful for Swift, it is not immediately apparent how
one would best implement this at this time.

...we disabled in swiftlang#81354

This requires a couple of supporting changes

* under Linux, do not cross compile by default -- that will ensure that
  the compiler-rt build will use the just built compiler and not the
system one (which may not be new enough for this purpose);
* provide sanitizer flags depending on the linker the just built compiler
 will use -- this detection is brittle, so ensure the user can override
those if needed.

Addresses rdar://150849329