Re: change "microcode: Use fallthrough attribute on clang 10 and 11 too." broke the build for me

[Taylor R Campbell]

> Date: Mon, 15 Feb 2021 00:17:45 -0800
> From: Chris Hanson <cph@chris-hanson.org>
> 
> >> I'll make the change and cherry-pick to release-11, unless you want to
> >> do it.
> > Go for it!
> 
> Done.

Thanks!

> I notice that there are several other microcode changes that seem 
> related to the new macs. Should those be cherry-picked too? If you agree 
> could you please select the relevant ones?
> 
> I'd like to release 11.2 with working support for the new macs.

Feel free to cherry-pick them, but unfortunately it's not that easy --
after making enough changes to get the microcode built, I learned that
W^X is now mandatory on macOS/aarch64.

So we need to teach the ucode (declare-compiled-code-block and gc) to
store compiled blocks in a separate PROT_READ|PROT_EXEC heap written
with the MAP_JIT API, and keep the main heap PROT_READ|PROT_WRITE.