dtrace: export dtrace_dis_get_byte()

kinst uses this function as well, but because it is not exported, it
implements its own copy of it. The patch also exposes the function to
userland, so programs that need to use dtrace_disx86() can use this
function instead of rolling their own copies.

Reviewed by: markj
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39871