linux: fix epoll_pwait() fallback on arm64

arm64 doesn't have a epoll_wait() system call but a logic error stopped
libuv from falling back to epoll_pwait().

This bug was introduced in commit libuv/libuv@67bb2b5 ("linux: fix
epoll_pwait() regression with < 2.6.19") which sadly exchanged one
regression for another.

This commit is a back-port of libuv/libuv@1d8332f and should help
get the ARMv8 buildbot in better shape.

Original-PR-URL: https://github.com/libuv/libuv/pull/308
PR-URL: https://github.com/iojs/io.js/pull/1365


Reviewed-By: Fedor Indutny <fedor@indutny.com>