kbuild/arch/xtensa: Define LINKER_SCRIPT for the linker script

Define the LINKER_SCRIPT when building the linker script as being done
in other architectures. This is required, because upcoming Makefile changes
would otherwise break things.

Signed-off-by: Nadav Amit <namit@vmware.com>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Chris Zankel <chris@zankel.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Michal Marek <michal.lkml@markovi.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-xtensa@linux-xtensa.org
Link: http://lkml.kernel.org/r/20181003213100.189959-2-namit@vmware.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>