Add aarch64a_standalone baremetal runtime variant

[pzhengqc]

This is to support the libc-standalone.a variant from musl-embedded.

[Zhaoshi Zheng (zhaoshiz)]

can you please explain what does this flag do?

[pzhengqc]

This is a multilib custom flag (https://clang.llvm.org/docs/Multilib.html#multilib-custom-flags) used only for multilib selection. We have already added a few other custom flag to help with multilib selection for variants like shadow call stack, multi-threading, etc.

[Jonathon Penix (jonathonpenix)]

I think it might be worth making this more generic to accommodate the uselocks and uselocks/standalone variants.

Doesn't need to be exactly this, but so we can have something like:

- Name: musl-subvariant
  Values:
    - Name: standalone
    - Name: uselocks
    - Name: no-subvariant
  Default: no-subvariant

(I don't necessarily mind if we do something like this when adding those either)

[pzhengqc]

Thanks for the suggestion. I've renamed according to your suggestion to accommodate other variants. I'll add an entry for uselock in a separate patch when adding the uselock variant.

[Jonathon Penix (jonathonpenix)]

LGTM, thank you!

[Ana Pazos (apazos)]

pzhengqc, let's hold off on this patch. Might not have users for it anylonger.

[Ana Pazos (apazos)]

Let's hold on from merging, no clear user of these variants.