[GWP-ASan] Removed unittests from Android build.

Summary:
Longstanding issues in the Android test runner means that compiler-rt unit
tests don't work on Android due to libc++ link-time issues. Looks like the
exported libc++ from the Android NDK is x86-64, even though it's part of the
ARM[64] toolchain... See similar measures for ASan and sanitizer-common that
disable unit tests for Android.

Should fully fix the Android bots (@vlad.tsyrklevich).

Reviewers: vitalybuka

Reviewed By: vitalybuka

Subscribers: srhines, kubamracek, mgorny, javed.absar, kristof.beyls, #sanitizers, llvm-commits, vlad.tsyrklevich

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D63019

llvm-svn: 362842