kboot: .note.GNU-stack is needed

Add '.section .note.GNU-stack,"",%progbits' to all assembler. Newer
versions of clang complain when this isn't present because executable
stacks are going away in the future. We don't need an executable stack
anyway.

Sponsored by: Netflix
Reviewed by: kevans, andrew, emaste, jhibbits
Differential Revision: https://reviews.freebsd.org/D49855