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_arm.go b/unix/syscall_linux_arm.go
index 8cf9e26..073a7a3 100644
--- a/unix/syscall_linux_arm.go
+++ b/unix/syscall_linux_arm.go
@@ -86,9 +86,16 @@
 //sys   Pread(fd int, p []byte, offset int64) (n int, err error) = SYS_PREAD64
 //sys   Pwrite(fd int, p []byte, offset int64) (n int, err error) = SYS_PWRITE64
 //sys	Truncate(path string, length int64) (err error) = SYS_TRUNCATE64
-//sys	Fadvise(fd int, advice int64, offset int64, length int) = SYS_FADVISE64
 //sys	Ftruncate(fd int, length int64) (err error) = SYS_FTRUNCATE64
 
+func Fadvise(fd int, offset int64, length int64, advice int) (err error) {
+	_, _, e1 := Syscall6(SYS_ARM_FADVISE64_64, uintptr(fd), uintptr(advice), uintptr(offset), uintptr(offset>>32), uintptr(length), uintptr(length>>32))
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
 //sys	mmap2(addr uintptr, length uintptr, prot int, flags int, fd int, pageOffset uintptr) (xaddr uintptr, err error)
 
 func Fstatfs(fd int, buf *Statfs_t) (err error) {