ar71xx/lzma-loader: fix O32 ABI conformance

ar71xx/lzma-loader: fix O32 ABI conformance

According to the calling convention of the o32 ABI the caller
function must reserve stack space for $a0-$a3 registers in case
the callee needs to save its arguments.

The assembly code of the loader does not reserve stack space for
these registers thus when the 'loader_main' function needs to save
its arguments, those will be stored in the 'workspace' area instead
of the stack.

Because the workspace area is also used by other part of the code, the
saved register values gets overwritten and this often leads to failed
kernel boots.

Fix the code to reserve stack space for the registers to avoid this
error.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Backport of r48279

git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@48852 3c298f89-4303-0410-b956-a3cf2f4a3e73