Fix HelloWorld attach test for Linux kernels with ptrace ancestor lockdown.

Similar to a recent test I fixed for gdb-remote attach scenarios, this
fix is for Linux kernels, such as Ubuntu's stock setup on 11.04-ish and
later, where ptrace starts requiring a ptracer to be an ancestor of the
inferior to be ptraced.  This change checks for Linux and the ptrace-related
flags.  If they're found, it tries to switch on the "allow any ptracer" mode
for the inferior as the first statements in the program.  It's a best-effort
solution - if the prctl call fails, the failure is ignored, and probably will
lead to the test failing.

The ptrace security behavior can be modified system-wide, but is outside the
scope of the test to address.  Hence I went with this particular solution.