machine: add Microblaze support for Zephyr toolchain (32-bit, little-endian)

[alpsayin]

Utilises the existing sources. Forked off of zephyr's picolibc mirror to be able to collect consistent results from its CI.

Helps meson pick the microblaze cpu family from microblazeel tool.
do-microblazeel-configure then picks cross-microblazeel-zephyr-elf.txt
zephyr only supports little-endian so far (last I checked), thus I never bothered to make Zephyr sdk-ng produce a big-endian toolchain. Hence no picolibc support for microblaze-zephyr-elf (big-endian is default).

No QEMU test set because QEMU invokation would need a -hw-dtb file such as
https://github.com/zephyrproject-rtos/zephyr/blob/713b955801c44aadf2e66ed972b83da8ca03d837/boards/microblaze/qemu_microblaze/board-qemu-microblaze-demo.dtb

But @keith-packard and I agree that, that's the next step. Let's see with this PR first if it builds.

Future work?

1. Vitis shipped microblaze-xilinx-elf toolchain support?

EDIT/Update: it seems there is no CI here.
EDIT/Update2: Of course there isn't. I'll open a draft PR in sdk-ng as usual.
EDIT/Update3: Future work.

[keith-packard]

How much testing has this seen? Did you actually try the machine-specific APIs on a target?

Only changes required are to not include this in CI (yet), and fix the copyrights to exclude me from the list.

[keith-packard]

When you've fixed the CI issues, I can let CI run, but it won't tell us much. Please let me know how much testing you've managed to do.

[alpsayin]

How much testing has this seen? Did you actually try the machine-specific APIs on a target?

1st iteration: None.
Current iteration b52120d: It builds and builds tests if I disable atomic-ungetc. I was also able to compile a no-stdio C program with the output specs.
Using the Zephyr SDK toolchain zephyr-sdk-0.16.0-beta1-7-g2a8106a from zephyrproject-rtos/sdk-ng#630
I'll try to load some of the test executables into a QEMU machine.
I may also write a run-microblaze script to automate testing the executables.

[alpsayin]

Did you actually try the machine-specific APIs on a target?

Could you elaborate on what you mean by machine-specific APIs?

[keith-packard]

How much testing has this seen? Did you actually try the machine-specific APIs on a target?

1st iteration: None. Current iteration b52120d: It builds and builds tests if I disable atomic-ungetc.

Hrm. What's the deal with atomic ungetc? Does the target have atomic operations of some size? Is it like RISC-V and only support atomic 32- and 64- bit operations? (there are code paths for that, btw). Without atomic ungetc, stdio isn't re-entrant as it doesn't otherwise need (or provide) any locking.

I was also able to compile a no-stdio C program with the output specs. Using the Zephyr SDK toolchain zephyr-sdk-0.16.0-beta1-7-g2a8106a from zephyrproject-rtos/sdk-ng#630 I'll try to load some of the test executables into a QEMU machine. I may also write a run-microblaze script to automate testing the executables.

That's awesome. If you manage that, and can do something like the arm semi-hosting (or even the x86 'e9' hack) you should be able to run the complete picolibc test suite without any code outside of picolibc. There's no requirement for merging to picolibc though; lots of hosts just don't have any semihosting support yet.

[keith-packard]

Did you actually try the machine-specific APIs on a target?

Could you elaborate on what you mean by machine-specific APIs?

Yes: all of the code in newlib/libc/machine/microblaze. None of that will get any testing using other targets, so some level of validation would be helpful, even if done just once by hand.

[keith-packard]

The spurious #endif in abort.c seems wrong to me?

[keith-packard]

We'll let CI run on this now; when that passes (which it should, unless we've missed something), I'll get it merged.

[alpsayin]

1. Rebased on main
2. Renamed scripts/do-microblaze-configure -> scripts/do-microblazeel-configure
3. Disabled fstack-protector for tests to stop build from moaning about unsupported option
4. while loop condition that is also an assignment is placed in parentheses to stop build warnings.
5. Fixed licenses.