Noticed when was building UNREG ghc with -optc{-Wall,-Werror}:

  rts/sm/Storage.c:1359:3: error:
     error: implicit declaration of function '__clear_cache'
       [-Werror=implicit-function-declaration]
       __clear_cache((void*)begin, (void*)end);
       ^~~~~~~~~~~~~
       |
  1359 |   __clear_cache((void*)begin, (void*)end);
       |   ^

Left direct '__clear_cache' usage gcc toolchain before 4.4.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>