Add a test exercising ptrace watchpoint functionality

This adds a test which verifies that watchpoints work on devices
which support them. It is motivated by issues experienced by Nexus
5 devices, where watchpoints work only sporadically.

These tests spawn a process, trace it, and verify it hits a
watchpoint of various sizes while executing on all available CPUs
(to help catch a recent bug, where some debugging functionality
was not available on non-boot CPUs). All x86(_64) targets are
expected to support this functionality. In case of arm(64),
watchpoints are expected to work on devices where the relevant
syscall indicates watchpoint presence. In case of mips, this test
is skipped as I could not verify that the test would work
correctly due to lack of hardware, and this test is intrinsically
hardware-dependend.

cherry picked from commit 1faca6c9

Bug: 28395519
Change-Id: Ie5cbb16b9eef51f322c02ed8dce712c21cad94cf