ARM: 6470/1: atomic64: use generic implementation for OABI configurations

The old apcs-gnu ABI doesn't guarantee that double words are allocated
to registers with even alignment, causing the 64-bit exclusive memory
operations to be rejected by the assembler.

This patch requires that CONFIG_AEABI is set in order to use the native
atomic operations and falls back to the generic (spinlock) code otherwise.

Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>