UNREG: use __builtin___clear_cache where available
UNREG: use __builtin___clear_cache where available

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);
       |   ^

Signed-off-by: Sergei Trofimovich <110fdadc76b6bce16977f568bdb4723eaed6fb3f@gentoo.org>

shlevy/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc