xtensa: fix linker script preprocessing - use LINKER_SCRIPT macro

Fix custom linker script preprocessing rule to define LINKER_SCRIPT
like the standard macro.  And use it in coprocessor.h to avoid both
C and assembler material in the linker script.

Signed-off-by: Marc Gauthier <marc@tensilica.com>
Signed-off-by: Chris Zankel <chris@zankel.net>