Runtime detection of android_set_abort_message.

Summary:
Use runtime detection (with a weak-undef symbol) of
android_set_abort_message availability. Android NDK provides a single
version of the ASan runtime library to be used for any target API
level, which makes compile-time feature detection impossible (the
library itself is built at API level 9).

Reviewers: vitalybuka

Subscribers: srhines, llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D37716

llvm-svn: 312973