x86/idt: Cleanup trap_init()

No point in having all the IDT cruft in trap_init(). Move it into the IDT
code and fixup the comments.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200528145522.992376498@linutronix.de