swift234

[CMake, build-script] Add support for cross-compiling tools.

This adds support to build-script and cmake scripts for cross-compiling the host tools in addition to stdlib.

To cross-compile, build-script-impl now accepts --cross-compile-tools-deployment-targets with a space separated list of targets to cross compile host Swift tools for.

For example: $ build-script ...other-args... -- --cross-compile-tools-deployment-targets="iphoneos-arm64 iphoneos-armv7".

When installing cross-compiled tools, it now also merges and runs lipo to produce fat binaries by merging the cross-compiled target architectures.

Swift SVN r24712