MacOS cross-compiler installation

[anjohnson]

In epics-base/epics-base#675 Jerzy showed how Homebrew can install a linux-x86_64 cross-compiler on a MacOS system:

brew tap messense/macos-cross-toolchains
brew install x86_64-unknown-linux-gnu

If the Core Group agrees it's worth supporting cross-builds on MacOS, ci-scripts should be able to install this.

[jerzyjamroz]

@anjohnson , I do not know if it is useful, but one can create templates for all these architectures straightforwardly (I personally use only: x86_64-unknown-linux-gnu):

aarch64-unknown-linux-gnu
aarch64-unknown-linux-musl
arm-unknown-linux-gnueabi
arm-unknown-linux-gnueabihf
arm-unknown-linux-musleabihf
armv7-unknown-linux-gnueabihf
armv7-unknown-linux-musleabihf
i686-unknown-linux-gnu
i686-unknown-linux-musl
mipsel-unknown-linux-gnu
x86_64-unknown-linux-gnu
x86_64-unknown-linux-musl