[sanitizer] Add i686 arch to compiler-rt.

This is needed so we can produce -i686- named libraries for
x86 Android (which is i686-linux-android).

An alternative solution would be keeping the "i386" name internally and
tweaking the OUTPUT_NAME of compiler-rt libraries.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@218761 91177308-0d34-0410-b5e6-96231b3b80d8