native_posix: override C standard version to 2011
native_posix: override C standard version to 2011

Override the C standard used for compilation to C 2011
This is due to some tests using _Static_assert which is a 2011
feature, but otherwise relying on compilers supporting it also
when set to C99.
This was in general ok, but with some host compilers and C
library versions it led to problems. So we override it to 2011
for native_posix board.

Fixes: #8529

Signed-off-by: Alberto Escolar Piedras <52be3174920ed06cbdfbcf9622a1a6c68e942617@oticon.com>

Vudentz/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,punitvara/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,kraj/zephyr,kraj/zephyr,galak/zephyr,ldts/zephyr,punitvara/zephyr,Vudentz/zephyr,punitvara/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,ldts/zephyr,galak/zephyr,galak/zephyr,punitvara/zephyr,ldts/zephyr,Vudentz/zephyr,nashif/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,explora26/zephyr,finikorg/zephyr,finikorg/zephyr,explora26/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,kraj/zephyr