linux: fix epoll_pwait() sigmask size calculation

Revisit the fix from commit b705b53.  The problem with using sigset_t
and _NSIG is that the size of sigset_t and the value of _NSIG depend
on what headers libuv picks up first, <signal.h> or <asm/signal.h>.
With the former, sizeof(sigset_t) = 128; with the latter, it's 8.

Simply sidestep the issue by calculating the signal mask as a 64 bits
integer, without using sigset_t or _NSIG.

This is a partial cherry-pick of commit 751ac48 from the v1.x branch.

Original PR-URL: https://github.com/libuv/libuv/pull/83

R=@saghul