Tracking: RISC-V platform guide documentation

[a4lg]

#1779 is once merged but reverted by #1792.

This is because rustdoc does not properly support multiple references to a single footnote and generates HTML with duplicate IDs.
The author proposed rust-lang/rust#140434 to enable writing such references without creating elements with the same ID.

TODO

Requirements (repository=rust)

• rustdoc: Allow multiple references to a single footnote rust#140434 is merged.
• stage0 (bootstrap) compiler is bumped and bumped compiler contains the change above.

What to do

• repository=stdarch: Re-apply [RFC] std_detect: RISC-V platform guide documentation #1779 with a minor fix (RISC-V: Remove footnote from the Zcb extension #1789)
• repository=rust: Update the stdarch submodule

cc: @sayantn