Exclude AES-GCM code for x86-32 due to heavy usage of 64-bit registers
not available on x86-32.

While at it, fixed unregister order in aesni_exit().
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>