[hwasan] Add entire report to abort message on Android.

Summary:
When reporting a fatal error, collect and add the entire report text to
android_set_abort_message so that it can be found in the tombstone.

Reviewers: kcc, vitalybuka

Subscribers: srhines, kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D54284

llvm-svn: 346557