toolchain: gcc: Add compiler barrier at the end of UNALIGNED_PUT()
toolchain: gcc: Add compiler barrier at the end of UNALIGNED_PUT()

compiler_barrier() is itself defined down in this file. Without
adding it, newer versions of GCC (7+) for ARM Cortex-M may mistakenly
coalesce multiple strb/strh/str (store byte/half-word/word)
instructions, which support unaligned access on some
sub-architectures (Cortex-M3 and higher, but not on Cortex-M0),
into strd (store double), which doesn't support unaligned access.

Fixes: #6307

Signed-off-by: Paul Sokolovsky <35cb557f011771c94f127d6fef99f804b62203f5@linaro.org>

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