gn build: Add check-hwasan target.

The Android sanitizer tests are currently some of the most difficult
to run correctly, requiring at least 3 build directories which have
to be configured in just the right way and built in the correct order
(see e.g. [1] and the functions that it calls).

This patch adds a check-hwasan target which greatly simplifies running
the hwasan tests for gn users, taking advantage of its support for
multiple toolchains. With this the tests can be run simply by setting
an NDK path and running "ninja check-hwasan" with a compatible Android
device connected. The Linux/x86_64 and Android/aarch64 targets are
tested in parallel.

[1] https://github.com/llvm/llvm-zorg/blob/master/zorg/buildbot/builders/sanitizers/buildbot_android.sh

Differential Revision: https://reviews.llvm.org/D56713

llvm-svn: 351277