Use arm-linux-android triple for arm32 runtimes

Bug: http://b/35404115

This also means that compiler-rt builtins are disabled for arm32.  Once
armv5 support in the NDK is dropped, we can target armv7 and re-enable
build of builtins.

Test: python toolchain/llvm/android/build.py
Change-Id: I509e5eabf2b26199380f3137f4729b603c0ea79d