UNREG: use __builtin___clear_cache where available

Authored by trofi on Jun 21 2017, 3:17 PM.

Description

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);
     |   ^

Left direct '__clear_cache' usage gcc toolchain before 4.4.

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