Add docs about SDKs and C compilation on armv7a-vex-v5

[lewisfm]

This PR expands the documentation for armv7a-vex-v5 to fix broken links, add more information about SDKs, and create a new section about C compilation.

I'm not super familiar with how Rust chooses what linker configuration to use / how the cc crate chooses what compiler to use - is this configuration to make C compilation work usually something that would be included in the compiler itself? Either way, it requires a lot of extra work which is kind of unfortunate (downloading a separate compiler, adding it to path, setting up a bunch of environment variables, and making a cargo config for linking).

[rustbot]

Some changes occurred in src/doc/rustc/src/platform-support

cc @Noratrieb

[rustbot]

r? @ehuss

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[ehuss]

I'd like to CC the other target maintainers just in case they have any feedback (if they haven't looked at this already): @Gavin-Niederman @max-niederman

Otherwise I'm fine with merging.

@lewisfm, the user @Tropix126 does not seem to exist. Do you happen to know if they are still around?

[Gavin-Niederman]

LGTM

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[tropicaaal]

@lewisfm, the user @Tropix126 does not seem to exist. Do you happen to know if they are still around?

Hi, that's me! My GitHub handle changed, but I'm still around. I'll have a look through this in a moment.

[tropicaaal]

LGTM

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[Noratrieb]

can you update the GitHub handle then @lewisfm? :)