In some xtensa configurations there may be system/user registers in
xtensa-modules with negative index. ISA initialization for such config
may clobber heap and result in program termination.
Don't update lookup table entries for register with negative indices.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
