arm: userspace: Do not overwrite r7 during syscall.

The r7 register is used as a frame pointer on ARM Thumb. As result, it
cannot be modified by the assembly code in functions using stack frame.

This commit replaces r7 by r8, which is a general purpose register.
Also it fixes #7704.

Signed-off-by: Piotr Zięcik <piotr.ziecik@nordicsemi.no>