__clear_cache: decorate with COMPILER_RT_EXPORT

Use COMPILER_RT_EXPORT rather than COMPILER_RT_ABI for this function.  Adding an
explicit PCS standard to the function causes a mismatch between the
declarations.  Furthermore, the function is implemented in C, and should take
the CC based on the target triple.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@208591 91177308-0d34-0410-b5e6-96231b3b80d8