rdrand_asm.S: On x86-64 we have enough registers, avoid repeated loads

rdrand_asm.S: On x86-64 we have enough registers, avoid repeated loads

On x86-64 there are enough registers that there really is no point in
using a repeated memory operand for the key material.  Load it into a
register instead, hopefully it will be slightly faster.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>