toolchain: gcc: Only use _Static_assert if building with C11
toolchain: gcc: Only use _Static_assert if building with C11

Zephyr currently builds as C99, whereas _Static_assert is a C11
feature.

Signed-off-by: Leandro Pereira <51c7437a02b34df685e6b4ecb2e9bca23efe2e1a@intel.com>

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