Using Darwin version number in platform name of macOS is misleading

[giordano]

There are some tools, and most notably the linker ld64, which expect the macOS version, and not the Darwin version, to be present in the platform name: https://github.com/apple-oss-distributions/ld64/blob/dbf8f7feb5579761f1623b004bd468bdea7c6225/src/ld/Options.cpp#L5478-L5500. #284 is a hot-fix to work around this problem, but if in the future we're going to reorganise the toolchain tree we may want to change this version number.