unix: don't fail TestPpoll on EINTR

TestPpoll sometimes fails builders due to Ppoll getting interrupted and
returning EINTR:

--- FAIL: TestPpoll (0.00s)
    syscall_linux_test.go:299: Ppoll: unexpected error: interrupted system call

Fix this by retrying Ppoll in case of EINTR, same as CL 298189 in
TestPoll.

Fixes golang/go#66324

Change-Id: I8ce4e2c00fe3b7a078cd75b4b15bb076d3a87fb2
Reviewed-on: https://go-review.googlesource.com/c/sys/+/627395
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>