ARM: Align stack to 8 bytes

The ARM ABI requires that the stack be aligned to 8 bytes as it is noted
in Procedure Call Standard for the ARM Architecture:
http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042d/index.html



Unaligned SP also causes the problem with variable-length arrays
allocation when VLA address becomes less than stack pointer during
aligning of this address, so the next 'push' in the stack overwrites
first 4 bytes of VLA.

Signed-off-by: Vitaly Kuzmichev <vkuzmichev@mvista.com>

Tested on tx25(mx25), imx27lite(mx27), qong(mx31) and trab(s3c2400)
Tested-by: Wolfgang Denk <wd@denx.de>