sys_ptrace_test: Better detect missing HW support

If the hardware debug support is missing from the CPU, the kernel can
end up in the state where it reports that it supports N(>0) watchpoints,
but the "maximum supported watchpoint size" is zero.

This is not a really sensible API, but it is what we have. I'm going to
speak to folks upstream about whether we can have it report something
more sensible, but that will take a while (if ever).

So, for the time being, detect this situation and treat it as if no
hardware watchtpoints/breakpoints are supported.

Test: bionic-unit-tests --gtest_filter="sys_ptrace.*"
Change-Id: Id6a013c9bec8d8b450b4a36c28431f6b7fd15879
Bug: 63116544