symm/rijndael-x86ish-aesni.S: Fix conflict in 32-bit register allocation.

symm/rijndael-x86ish-aesni.S: Fix conflict in 32-bit register allocation.

Since 28321c9..., the context pointer in ECX was overwritten with the
GOT pointer, used to find the end-swapping table, resulting in an abort.
Reallocate so that the round count, which is loaded /after/ the endswap
pointer, ends up in ECX, and the context pointer goes in EAX, which
doesn't get clobbered.

This doesn't affect 64-bit targets.