Allow multiple FBT probes to share a tracepoint.

With GNU ifuncs, multiple FBT probes may correspond to the same
instruction. fbt_invop() assumed that this could not happen and
would return after the first probe found in the global FBT hash
table, which might not be the one that's enabled. Fix the problem
on x86 by linking probes that share a tracepoint and having each
linked probe fire when the tracepoint is hit.

PR: 230846
Approved by: re (gjb)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D16921