unix: fix build for linux/386 and linux/arm

Because ARM ABI requires 64-bit argument to be passed in even
register pairs, to avoid wasting one register between fd and
advise in the usual fadvise64 syscall signature, linux/arm
has its own variation that reorders the argument so that the
arguments fit in six registers.

While we're at it, also fix build for linux/386.

Fixes golang/go#10294.

Change-Id: I322e2226619c5aa9c096a1d5cb7ae1e94fd4a5a1
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/8282
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/unix/syscall_linux_amd64.go b/unix/syscall_linux_amd64.go
index 6690e0d..7a89584 100644
--- a/unix/syscall_linux_amd64.go
+++ b/unix/syscall_linux_amd64.go
@@ -7,6 +7,7 @@
 import "syscall"
 
 //sys	Chown(path string, uid int, gid int) (err error)
+//sys	Fadvise(fd int, offset int64, length int64, advice int) (err error) = SYS_FADVISE64
 //sys	Fchown(fd int, uid int, gid int) (err error)
 //sys	Fstat(fd int, stat *Stat_t) (err error)
 //sys	Fstatfs(fd int, buf *Statfs_t) (err error)