xtensa: Setup CROSS_COMPILE at the top

CROSS_COMPILE must be setup before using e.g. cc-option (and a few other
as-*, cc-*, ld-* macros), else they will check against the wrong compiler
when cross-compiling, and may invoke the cross compiler with wrong or
suboptimal compiler options.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Chris Zankel <chris@zankel.net>