[ARM] 3210/1: add missing memory barrier helper for NPTL support

Patch from Nicolas Pitre

Strictly speaking, the NPTL kernel helpers are required for pre ARMv6
only.  They are available on ARMv6+ as well for obvious compatibility
reasons.  However there are cases where extra memory barriers are needed
when using an SMP ARMv6 machine but not on pre-ARMv6.

This patch adds a memory barrier kernel helper that glibc can use as
needed for pre-ARMv6 binaries to be forward compatible with an SMP
kernel on ARMv6, as well as the necessary dmb instructions to the
cmpxchg helper.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Acked-by: Daniel Jacobowitz <dan@codesourcery.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>