Implement Android versions of __clear_cache for arm32 & mips.

The MIPS version was reviewed by myself. The ARM version was reviewed by JF
Bastien.

Patch by Narayan Kamath <narayan@google.com>.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@203674 91177308-0d34-0410-b5e6-96231b3b80d8