bionic: Add magic to abort message

This change adds a 128-bit magic value just before the abort message
(and its length). This allows Chrome OS crash collector (which cannot
reliably correlate a crash with its corresponding tombstone) to extract
the abort message from the core dump.

Bug: 62387553
Test: /data/nativetest{,64}/debuggerd_test/*
Test: /data/nativetest{,64}/bionic-unit-tests{,-static}/*

Change-Id: I69ff021d773b3a10081ab16ddf6f23233320fb02