selftests/powerpc: Ignore generated files

Currently some binary files which are generated when tests are compiled
are not ignored by git, so 'git status' catch them.

For copyloops test, fix wrong binary names already in .gitignore. For
ptrace, security, and stringloops tests add missing binary names to the
.gitignore file.

Signed-off-by: Gustavo Romero <gromero@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190814205638.25322-2-gromero@linux.ibm.com