xtensa: disable link optimization

The default linker behavior is to optimize identical literal values
and remove unnecessary overhead. However, because of a bug in the
linker, this currently results in an error ('call target out of range').

Disable link-time optimizations per default until there is a fix
for the linker and add the option to iss_defconfig.

Signed-off-by: Chris Zankel <chris@zankel.net>