selftest, x86: fix incorrect comment

The current comment indicates it's checking for a 32-bit build
environment, but it actually checks for a 64-bit environment. Fix this.

Signed-off-by: Martin Kelly <martkell@amazon.com>
Acked-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>