Formally drop support for cygwin32

N.B. During development in !9263 (merged) we came across multiple tasks and bugs regarding the configuration script. In preparation of merging it, we're opening tickets for the leftover tasks both regarding the new ghc-toolchain and the toolchain logic in configure. This is one of those tickets.

Currently the cygwin32 operating system is recognized by the configure script. However:

• we have not tested support for Cygwin in many, many years
• I have strong doubts that it actually works
• building GHC via Cygwin (which is essentially a crude UNIX emulation layer) when we have first-class Windows support seems like a bad idea

I suggest that we formally declare this configuration to be unsupported.