*N.B. During development in !9263 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.* While writing `ghc-toolchain` we noticed that the `FP_PROG_LD_NO_COMPACT_UNWIND` check is subtly wrong. Specifically, we pass `-Wl,-no_compact_unwind` to `cc`. However, `ld.gold` interprets this as `-n o_compact_unwind`, which is a valid argument. We should just make this check specific to Darwin (and mirror this change in `GHC.Toolchain.Tools.Link.checkSupportsCompactUnwind`).