Assert that sysroot ABI version matches exactly

Otherwise, fall back to the multi ABI scheme, except in testing, where
it becomes a hard error.

This should make it possible to use a rustup-provided rust-analyzer with
proc macro dylibs compiled by older rustcs, and it'll also catch changes
to the format of `rustc --version` or the `.rustc` section that would
make them impossible to compare for equality.

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