Add setjmp/ucontext definitions for FreeBSD/aarch64

[valpackett]

Tested with ldc ltsmaster.

Relevant headers:
https://github.com/freebsd/freebsd/blob/master/sys/arm64/include/setjmp.h
https://github.com/freebsd/freebsd/blob/master/sys/arm64/include/ucontext.h

[dlang-bot]

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + druntime#2269"

[joakim-noah]

Looks like this new submitter needs to be approved on the auto-tester before it will run.

[wilzbach]

Approved (:

[wilzbach]

Can we please use the new alias syntax? (will correct this quickly myself)

[wilzbach]

ERROR: Permission to myfreeweb/druntime.git denied to wilzbach.

Oh well, it looks like you need to do it yourself or enable maintainer access to your PRs.

[valpackett]

Oh the one with the = sign is new? I'm not familiar with D, this is the first D code I'm editing :) I assumed this is the new one since it's more common in that file.

[wilzbach]

yes, alias __register_t = long is the "new alias syntax".
Unfortunately the old one was never deprecated and as there's still some code that uses it (druntime being a prime example).
We try to avoid using it for new code at least ;-)
Thanks for the fix-up!