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

[christos]

Summary

Currently kinst checks if only the first instruction is 'push %rbp',
essentially excluding functions that do push RBP, but not in the first
instruction.

[christos]

Check for 'pop %rbp' as well.

[markj]

sys/cddl/dev/kinst/amd64/kinst_isa.c

515–516

The first sentence isn't really accurate anymore. I'd just replace the comment with something like, "refuse to instrument functions lacking the usual frame pointer manipulations since they might correspond to exception handlers."

[christos]

Address Mark's comment.

[markj]

Please note in the commit message that this behaviour matches that of FBT. That is, FBT will put an entry probe on any function that contains a push %rbp.