aesni: Avoid loading AES/GHASH round keys into local variables

aesni: Avoid loading AES/GHASH round keys into local variables

The performance impact is not measurable, as the compiler loads these variables
in xmm registers in unrolled loops anyway.

However, we avoid loading these sensitive keys onto the stack. This happens for
larger key schedules, where the register count is insufficient. If that key
material is not on the stack, we can avoid to wipe it explicitly after
crypto operations.