unix: export KexecFileLoad on linux/arm

CL 175158 added the generated kexecFileLoad wrapper but forgot to add
the exported KexecFileLoad and the corresponding //sys line.

Change-Id: Ic94ce83dbb1ecc3d07080b4b4dfe35934ec54d57
Reviewed-on: https://go-review.googlesource.com/c/sys/+/175657
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_linux_arm.go

@@ -272,3 +272,16 @@ func SyncFileRange(fd int, off int64, n int64, flags int) error {
 	// order of their arguments.
 	return armSyncFileRange(fd, flags, off, n)
 }
+
+//sys	kexecFileLoad(kernelFd int, initrdFd int, cmdlineLen int, cmdline string, flags int) (err error)
+
+func KexecFileLoad(kernelFd int, initrdFd int, cmdline string, flags int) error {
+	cmdlineLen := len(cmdline)
+	if cmdlineLen > 0 {
+		// Account for the additional NULL byte added by
+		// BytePtrFromString in kexecFileLoad. The kexec_file_load
+		// syscall expects a NULL-terminated string.
+		cmdlineLen++
+	}
+	return kexecFileLoad(kernelFd, initrdFd, cmdlineLen, cmdline, flags)
+}