Description: 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

Index: b/includes/stg/MiscClosures.h
===================================================================
--- a/includes/stg/MiscClosures.h
+++ b/includes/stg/MiscClosures.h
@@ -390,8 +390,12 @@ RTS_FUN_DECL(stg_copySmallMutableArrayzh
 RTS_FUN_DECL(stg_casSmallArrayzh);
 
 RTS_FUN_DECL(stg_newMutVarzh);
+#if __GLASGOW_HASKELL__ < 808
+RTS_FUN_DECL(stg_atomicModifyMutVarzh);
+#else
 RTS_FUN_DECL(stg_atomicModifyMutVar2zh);
 RTS_FUN_DECL(stg_atomicModifyMutVarzuzh);
+#endif
 RTS_FUN_DECL(stg_casMutVarzh);
 
 RTS_FUN_DECL(stg_isEmptyMVarzh);