Drop `isARM` predicate

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 ghc-platform exposes GHC.Platform.ArchOS.isARM, which returns True for both ARMv7 and ARMv8 architectures. However, these are very different architectures and my sense is that it would be better to treat them individually; lumping them together will only lead to confusion.

Let's inline the predicate into its (few) use-sites and drop it.