Squashed 'site_scons/' changes from f062937..58df9bc

58df9bc Add Darwin platform and fix iisues with coverage and multiple toolchains
09513b5 Add the Darwin platform
c229c0b Create the toolchain env as a clone of the default env

git-subtree-dir: site_scons
git-subtree-split: 58df9bc6d15d3e26c727f4a81f61a1db8aff7cc2
Merge commit '7e5150d6f78bf1c1220a3e3ffe5a30abf4530e93'