runtime: fix Linux build

Make the definition of the EpollEvent data field consistent
across architectures, adapt the other use of it in
netpoll_epoll for the new definition, and use uint64 rather
than uintptr.

LGTM=dave
R=rsc, dave
CC=golang-codereviews
https://golang.org/cl/137890043

src/pkg/runtime/defs_linux_386.h

@@ -204,7 +204,7 @@ struct Itimerval {
 };
 struct EpollEvent {
 	uint32	events;
-	uint64	data;
+	byte	data[8]; // to match amd64
 };
 
 

src/pkg/runtime/defs_linux_arm.h

@@ -163,6 +163,6 @@ typedef struct EpollEvent EpollEvent;
 struct EpollEvent {
 	uint32	events;
 	uint32	_pad;
-	uint64	data;
+	byte	data[8]; // to match amd64
 };
 #pragma pack off

src/pkg/runtime/netpoll_epoll.c

@@ -37,7 +37,7 @@ runtime·netpollopen(uintptr fd, PollDesc *pd)
 	int32 res;
 
 	ev.events = EPOLLIN|EPOLLOUT|EPOLLRDHUP|EPOLLET;
-	*(uintptr*)ev.data = (uintptr)pd;
+	*(uint64*)ev.data = (uint64)(uintptr)pd;
 	res = runtime·epollctl(epfd, EPOLL_CTL_ADD, (int32)fd, &ev);
 	return -res;
 }
@@ -95,7 +95,7 @@ retry:
 		if(ev->events & (EPOLLOUT|EPOLLHUP|EPOLLERR))
 			mode += 'w';
 		if(mode)
-			runtime·netpollready(&gp, (void*)ev->data, mode);
+			runtime·netpollready(&gp, (void*)(uintptr)*(uint64*)ev->data, mode);
 	}
 	if(block && gp == nil)
 		goto retry;