[XTENSA] Fix makefile to work with binutils-2.18.

When building with binutils-2.18, vmlinux includes .note.gnu.build-id
sections that need to be stripped out when building the binary image.
The old .xt.insn sections haven't been used for a long time, so don't
bother stripping them.

Signed-off-by: Bob Wilson <bob.wilson@acm.org>
Signed-off-by: Chris Zankel <chris@zankel.net>