Move instructions into the arm64 exception vectors

[andrew]

Summary

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.

[markj]

What's the advantage of setting up the handlers this way?

sys/arm64/arm64/exception.S

39

274

Why is it not vempty 0?

[andrew]

Fix vempty exception level values

[kib]

In D33751#763217, @markj wrote:

What's the advantage of setting up the handlers this way?

I expect any reasonable implementation to prefetch the whole cache line from the exception vector, on exception. Using a single instruction there wastes cache.