Add "imprecise" watchpoint ptrace test

This tests for the presence of a kernel bug that meant that the kernel
would sometimes fail to report the watchpoint hit if the hardware
reported a address which did not exactly match the address range being
watched (which it is allowed to do per ARM spec if the instruction
accesses a larger block of memory than the region being watched). This
bug was fixed in linux kernel 4.9, and has been backported to older
android kernels.

Bug: 30802222
Bug: 30919905
Test: bionic-unit-tests --gtest_filter="sys_ptrace.*"
Change-Id: I80c35b29eaf28e2dbacb9e8ee5317fdea653fc87