x86: fix exec mappings comments

- noexec32 is on by default for years already
- add noexec32 to kernel-parameters and fix noexec typo in there

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>