Rollup merge of #49563 - japaric:std-thumb, r=alexcrichton

add a dist builder to build rust-std components for the THUMB targets

the rust-std component only contains the core and compiler-builtins (+c +mem) crates

cc #49382

- I'm not entirely sure if this PR alone will produce rust-std components installable by rustup or if something else needs to be changed
- I could have done the THUMB builds in an existing builder / image; I wasn't sure if that was a good idea so I added a new image
- I could build other crates like alloc into the rust-std component but, AFAICT, that would require calling Cargo a second time (one for alloc and one for compiler-builtins), or have alloc depend on compiler-builtins (#49503 will perform that change) *and* have alloc resurface the "c" and "mem" Cargo features.

r? @alexcrichton

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