[asan] Support ASAN_ACTIVATION_OPTIONS.

This is mostly useful for testing, as the only other way of specifying
activation options (Android system property) is system-wide and affects
concurrently running tests.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@224824 91177308-0d34-0410-b5e6-96231b3b80d8