Rework the arm64 sigtramp code to be a trampoline

[andrew]

Summary

To help with switching to a vdso sigtramp switch to passing through the
sigcode function when entering a signal. This ensures the return address
is within the function.

[kib]

sys/arm64/arm64/exec_machdep.c

605

When does this case needed at all?

[andrew]

sys/arm64/arm64/exec_machdep.c

605

It's used in Linux emulation.

[kib]

sys/arm64/arm64/exec_machdep.c

605

Don't Linux emulation need its own sendsig() implementation?

I doubt that our sigframe format is identical to the Linux one. Even, if unlikely it is, there is high probability that we diverge in future.

[andrew]

Rebase past removing the sv_sigcode_base check