Add ppc64 as a supported platform in npm test

npm-test-platform-all tests npm platform
restrictions by supplying all supported
platforms as valid platforms in the package.

This fails on ppc64 because the test doesn't
think ppc64 is a valid platform.

This change adds "ppc64" to the list of supported
platforms in the package.json

(Related is the equivalent for ppc in
commit 9a7d362366fa2de1ac9143648c0e895938f67cd1)