Minor tidyings to the debugger-attach code, as part of a failed

Minor tidyings to the debugger-attach code, as part of a failed
attempt to fix debugger attach on ppc32-linux and ppc64-linux (see
#151908).  The fork/ptrace-based mechanism works fine for x86-linux
and amd64-linux but not on ppc.  I have no idea what is going on.

It seems like the forked child process (to which we will attach GDB)
does not stop when it does PTRACE_TRACE_ME and so things go downhill
very rapidly after that.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@7184