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.

[giordano]

As a counter-example, GCC expects the Darwin version (which is probably why it was initially used here): https://github.com/gcc-mirror/gcc/blob/20b8779ea9bd82b26eeb195b30f695168cd7ae1d/libgcc/config.host#L237 (see for example https://en.wikipedia.org/w/index.php?title=MacOS&oldid=1266860412#Timeline_of_releases for a darwin <-> macOS correspondence table). Life is hard.