cxxabi: declare abi after __cxxabiv1 namespace

Fixes clang build

Change-Id: Ibf78ee5434387d9b89748431548cd41c6083811d
Merge "cxxabi: declare abi after __cxxabiv1 namespace"
Merge https://android.googlesource.com/platform/abi/cpp into jb-devel-mr2-release