syscall: use 64-bit file size for truncation on Linux

Truncate and Ftruncate for 32-bit Linux should use 64-bit
offset system calls.

R=rsc
CC=golang-dev, royger
https://golang.org/cl/2943041

src/pkg/syscall/syscall_linux_amd64.go

@@ -8,6 +8,7 @@ package syscall
 //sys	Fchown(fd int, uid int, gid int) (errno int)
 //sys	Fstat(fd int, stat *Stat_t) (errno int)
 //sys	Fstatfs(fd int, buf *Statfs_t) (errno int)
+//sys	Ftruncate(fd int, length int64) (errno int)
 //sys	Getegid() (egid int)
 //sys	Geteuid() (euid int)
 //sys	Getgid() (gid int)
@@ -32,6 +33,7 @@ package syscall
 //sys	Stat(path string, stat *Stat_t) (errno int)
 //sys	Statfs(path string, buf *Statfs_t) (errno int)
 //sys	SyncFileRange(fd int, off int64, n int64, flags int) (errno int)
+//sys	Truncate(path string, length int64) (errno int)
 //sys	accept(s int, rsa *RawSockaddrAny, addrlen *_Socklen) (fd int, errno int)
 //sys	bind(s int, addr uintptr, addrlen _Socklen) (errno int)
 //sys	connect(s int, addr uintptr, addrlen _Socklen) (errno int)