arm: put CONFIG_RCU_ARM_HAVE_DMB in the global config.h

Change CONFIG_ARM_HAVE_DMB to CONFIG_RCU_ARM_HAVE_DMB while at the
same time making it seen as a global setting in urch/config.h.

The initial ARM < v7 work missed this setting, which allowed the URCU
to package to build correctly, but exposed the memorry barriers
defines incorrectly for other applications using the URCU headers.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>