arm: Make arch specific memcpy thumb-safe.

arm: Make arch specific memcpy thumb-safe.

The current arch implementation of memcpy cannot be called
from thumb code, because it does not use bx instructions on return.
This patch addresses that. Note, that this patch does not touch
the hot loop of memcpy, so performance is not affected.

Tested on MXS (arm926ejs) with and without thumb-mode enabled.

Signed-off-by: Klaus Goger <klaus.goger@theobroma-systems.com>
Signed-off-by: Christoph Muellner <christoph.muellner@theobroma-systems.com>