Add FreeBSD x86-64 support to runtime trap handler

[mashtizadeh]

This patch was not discussed in a PR. This is a small change implement support for FreeBSD x86-64.

[alexcrichton]

Thanks for this! Ideally we wouldn't need a C shim here, so we're currently being careful about upstreaming support necessary into the libc crate. Can you be sure to do that so we can drop the C shim once libc is published and we update to that?

[valpackett]

mcontext for libc is in this patch: rust-lang/libc#1630

[alexcrichton]

Great! In that case mind rebasing this and I think we can merge?

[cfallin]

I'm cleaning up old PRs and I think we build cleanly on FreeBSD now; I'll go ahead and close this. If there are continued issues please do feel free to open PRs/issues as needed!