[GWP-ASan] Use weak abort message definition.

Summary:
New shard out of the Check() function for GWP-ASan uses
android_set_abort_message. This is happily present on bionic Android,
but not Android for glibc host x86. Fix up to use the weak definition
always, so we don't have to worry.

Reviewers: eugenis

Reviewed By: eugenis

Subscribers: #sanitizers, llvm-commits, pcc, cferris

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D74142