elf: Add EM_NANOMIPS value as a valid one for e_machine field

Value 249 is registered as valid for usage for nanoMIPS executables.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Signed-off-by: Stefan Markovic <smarkovic@wavecomp.com>