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 <paul.sokolovsky@linaro.org>