logging: Added config to disable vla in statements

The config LOG_USE_VLA depends on that MISRA_SANE is not set.
Unfortunately the IAR toolchain can't handle vla:s inside a
statement, which the LOG_USE_VLA macro does.

I've added a new, hidden config to check if vla is ok inside
statements, SUPPORT_VLA_IN_STATEMENTS. Default is always y, but
a toolchain can override it so that VLA is not used by
Z_LOG_MSG_ON_STACK_ALLOC.

Signed-off-by: Lars-Ove Karlsson <lars-ove.karlsson@iar.com>