kinst: check for 'push %rbp' anywhere in the function

Currently kinst checks if only the first instruction is 'push %rbp',
essentially excluding functions that do push RBP, but not in the first
instruction. This patch modifies kinst to check for 'push %rbp', as
well, as a following 'pop %rbp', anywhere in the function. This behavior
also matches that of FBT.

Reviewed by: markj
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40283