perlasm/x86_64-xlate.pl: add commentary.

   push @$opcode,($rex|0x40) if ($rex);

}

# older gas and ml64 don't handle SSE>2 instructions

# Upon initial x86_64 introduction SSE>2 extensions were not introduced

# yet. In order not to be bothered by tracing exact assembler versions,

# but at the same time to provide a bare security minimum of AES-NI, we

# hard-code some instructions. Extensions past AES-NI on the other hand

# are traced by examining assembler version in individual perlasm

# modules...

my %regrm = (	"%eax"=>0, "%ecx"=>1, "%edx"=>2, "%ebx"=>3,

		"%esp"=>4, "%ebp"=>5, "%esi"=>6, "%edi"=>7	);