ovs-atomic-msvc: Fix 64 bit atomic read/writes.

MSVC converts 64 bit read/writes into two instructions (uses 'mov' as
seen through cl //FAs). So there is a possibility that an interrupt can
make a 64 bit read/write non-atomic even when 8 byte aligned. So we cannot
use a simple assignment. Use a full memory barrier function instead.

Reported-by: Jarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>