x86-64, syscall: Adjust comment spacing and remove typo

Adjust spacing for comment so that it matches the multiline comment
style used in the rest of the kernel, and remove word duplication.

It is not really clear what version of gcc this refers to, but the
extra & doesn't cause any harm, so there is no reason to remove it.

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