overlays/xtensa/mtk: Fixup and normalize file paths

The new directory structure in the upstream Cadence tarballs wasn't as
compatible as I thought.  Binutils will produce a working assembler,
but only if it does NOT see the new xtensa-config.h (which was added
in a path it doesn't look at).  Also gcc ends up hitting the
in-tree/non-overlayed xtensa-config.h and produces a big endian
compiler!

Don't be fancy.  Move stuff around to match the older overlays
exactly.

And I'd forgotten the copy of core-isa.h for newlib (which SOF doesn't
use, but should still be present in the SDK).

Signed-off-by: Andy Ross <[email protected]>

Author: andyross