amd64: Add MD bits for KASAN

• Initialize KASAN before executing SYSINITs.
• Add a GENERIC-KASAN kernel config, akin to GENERIC-KCSAN.
• Increase the kernel stack size if KASAN is enabled. Some of the ASAN instrumentation increases stack usage and it's enough to trigger stack overflows in ZFS.
• Mark the trapframe as valid in interrupt handlers if it is assigned to td_intr_frame. Otherwise, an interrupt in a function which creates a poisoned alloca region can trigger false positives.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29455