Move instructions into the arm64 exception vectors

We have 32 instructions in each exception vector on arm64. Previously
only one was used to branch to the handler function. We can split the
start of these functions and move some of the instructions into the
vectors.

Reviewed by: kib, markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33751

(cherry picked from commit 77402d2834e64a3530fbaa47d57eccce23c58603)