rc4-x86_64.pl: commentary update.

# is better) and improvement coefficients relative to previous

# version of this module are:

#

# Opteron	5.3/+0%

# Opteron	5.3/+0%(*)

# P4		6.5

# Core2		6.2/+15%(*)

# Core2		6.2/+15%(**)

# Westmere	4.2/+60%

# Sandy Bridge	4.2/+120%

# Atom		9.3/+80%

#

# (*)	Note that Core2 result is ~15% lower than corresponding result

# (*)	But corresponding loop has less instructions, which should have

#	positive effect on upcoming Bulldozer, which has one less ALU.

#	For reference, Intel code runs at 6.8 cpb rate on Opteron.

# (**)	Note that Core2 result is ~15% lower than corresponding result

#	for 32-bit code, meaning that it's possible to improve it,

#	but more than likely at the cost of the others (see rc4-586.pl

#	to get the idea)...