Allow unregisterised ghc-8.6 to build newer GHC

Allow unregisterised ghc-8.6 to build newer GHC

Commit af9b744bbf1 introduced a regression stopping existing unregisterised
compilers from being able to compile newer versions of GHC. The problem is
that the bootstrap compiler uses the newer `includes/stg/MiscClosures.h` file
where some defines have been renamed, resulting in the following error:
.
 error: ‘stg_atomicModifyMutVarzh’ undeclared (first use in this function); did you mean ‘stg_atomicModifyMutVar2zh’?
.
 For more information, see https://gitlab.haskell.org/ghc/ghc/issues/17111.
.
 This patch can be removed, once ghc-8.6 is no longer the bootstrap compiler.
Author: Ilias Tsitsimpis <iliastsi@debian.org>
Bug: https://gitlab.haskell.org/ghc/ghc/issues/17111

Gbp-Pq: Name fix-build-using-unregisterised-v8.6